Memory controller having flip-flops for synchronously generating DRAM address and control signals from a single chip

ABSTRACT

A memory controller which makes maximum use of any processor pipelining and runs a large number of cycles concurrently. The memory controller takes advantages of different speed memory devices by operating each memory device at optimal speed. The functions are performed by a plurality of simple, interdependent state machines, each responsible for one small portion of the overall operation. As each state machine has completed its function, it notifies a related state machine that it can now proceed and thereafter waits for its next start or proceed indication. The next state machine operates in a similar fashion. The state machines responsible for the earlier portions of a cycle have started their tasks on the next cycle before the state machines responsible for the later portions of the cycle have completed their tasks. The memory controller is logically organized as three main blocks, a front end block, a memory block and a host block, each being responsible for interactions with its related bus and components and interacting with the various other blocks for handshaking. The memory system includes a single chip comprising a plurality of flip-flops which provides all of the address and control signals to a memory device so that a clock cycle can be saved because of reduced skew of the signals. These flip-flops are synchronously clocked by a common clock and provide the final output of the address and control signals for the DRAM devices.

This is a continuation of application Ser. No. 08/034,287 filed on Mar.22, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to memory controllers used with computers, andmore particularly to memory controllers providing address, row addressstrobe (RAS), column address strobe (CAS) and write enable signals todynamic random access memories (DRAMS).

2. Description of the Related Art

Microprocessor-based computer systems have been increasing inperformance at a tremendous rate. Much of this increase has been basedon the improvements in the microprocessor itself. For example, clockspeeds are reaching those previously used only by mainframe computers.However, affordable memory device performance has not been increasing atthe same rate. Indeed, dynamic random access memory (DRAM) performancehas flattened out recently, with the majority of the effort beingconcentrated on increasing device storage size. Thus main memory hasbecome a bottleneck.

Cache memory systems, where a small amount of very fast, expensivestatic RAM is used to store copies of the data, have made the problemsomewhat less severe, but the designs are very complicated andexpensive. Further, the poor memory performance returns when access mustbe made to main memory. So there still is a need to improve theperformance of the main memory system.

Page mode memory devices provide one way to increase memory systemperformance. If consecutive accesses are made to the same row address,referred to as the same page, only column addresses need be provided.This allows a dramatic reduction in cycle time for those cases, referredto as page hits. This is a quick gain and relatively easily made byitself, but more performance is always desired.

In many processors, the address is provided before the data portion ofthe cycle. This is referred to as pipelining and allows a performanceincrease in the memory systems. All of the address decoding can bestarted before the data portion begins, so that partially paralleloperations can occur.

In most cases this overlap is somewhat minimal, one or two clock cycles,as the related systems cannot handle much more complexity. For example,the memory controller is conventionally built as a synchronouscontroller, based on a major state machine to control events andadvancing on the processor clock edge. As the complexities of themicroprocessor and the computer system increase, the conventional memorycontroller design becomes very limiting. It is exceedingly difficult tobuild a major state machine to handle all the possible cases. Thecomplexity is daunting, if not overwhelming. As a result, numerouspotential performance improvements have to be limited. Usually thismeans that any pipelining is limited and there are very few paralleloperations. While some designs allowed write operations to be posted,this just further increased complexity in other ways. As a result, someother feature was sacrificed, so potential gains were lost.

Memory system performance is also a trade off between cost and speed.While conventionally 80 ns DRAMs have been used, 60 ns devices areavailable, though at a slightly higher cost. While prior memorycontrollers could utilize differing speeds of DRAMs, allowing the userto make the speed versus cost tradeoff, a mixed speed system did notobtain any benefits. The memory controller could use different speedDRAMs, but only one actual speed of operation was allowed in the system.The memory controller thus ran at the speed of the slowest of theinstalled DRAMs. This did not allow the user to have fast memory areas,such as the base memory area, and slow memory areas, such as extendedmemory locations in the main system memory controlled by the memorycontroller. This limitation arose again because of the complexities ofmemory controllers.

One further limitation to memory system design has been timing skew andpropagation delay problems in providing the actual address, RAS, CAS,and write enable signals to the memory devices. For DRAMs the address isprovided in two portions, the row address and the column address. Theseaddresses are typically developed by multiplexing the addresses providedto the memory system. A row address strobe or RAS signal is providedwith the row addresses, while a column address strobe or CAS signal isprovided with the column addresses. The particular RAS and CAS signalsprovided to individual devices are generally prepared from mastersignals, referred to as MRAS and MCAS. The master signals have thedesired timing information in relation to the addresses and data, butthey must be qualified by other signals so that only the proper memorydevices are activated. Generally this is done by doing bank selectionencoding of the MRAS signal, so that only the memory devices having inthe proper address range receive RAS signals. The MCAS signal istypically encoded with the signals relating to the particular byte orbytes of the full width which are active. In read cases this is usuallythe full width, but in write cases must be properly qualified so thaterroneous data is not written. Similarly, a master write enable or MWEsignal is used and provided to the devices. This signal can be providedto all of the devices as the CAS signal is providing the necessary chipenabling signal.

Conventionally the address multiplexing has been performed in one devicealong with master signal development, such as the memory controller chipitself, while the various additional qualifying has been done on otherchips external to the memory controller chip. In fact, the qualificationhas generally been done asynchronously, while some of the otherdevelopment has been done synchronously. One problem with thisarrangement is that with all of these operations, the multiplexing, themaster signal development, the further qualifications, and the mixedasynchronous and synchronous development, timing skews and propagationdelays become significant and must be considered when any timing marginanalysis is performed. Indeed, when working with the very high clockrates present in current microprocessor systems, the skews and delayscan add an entire clock cycle to each memory cycle, thus degradingsystem performance.

Thus there are memory system performance gains that could be achieved,but conventional design limitations render them only potential, notpractical. Therefore it is clearly desirable to have a memory controllerwhich makes maximum use of processor address pipelining, can runnumerous cycles concurrently in the greatest number of cases, caneffectively use different speed memory devices and minimizes any timingrelated problems in providing the various signals to the memory devices.

SUMMARY OF THE PRESENT INVENTION

A memory controller according to the present invention makes maximum useof any processor pipelining and allows a large number of cycles, up tofive, to be running concurrently on the computer system. This allows amarked improvement in the performance of the memory system.

The primary way this improvement is obtained is by moving the memorycontrol functions from a large, complicated master state machine to aplurality of simple, interdependent state machines. Basically each statemachine is responsible for one small portion of the overall operation,generally one specific function. As each state machine reaches a pointwhere it has completed its function, it notifies a related state machinethat it can now proceed on its task and proceeds to wait for its nextstart or proceed indication. The second state machine operates in asimilar fashion. This is repeated until a complete cycle is performed.But the state machines responsible for the earlier portions of a cycle,such as address and command decode, have started their tasks on the nextcycle before the state machines responsible for the later portions ofthe cycle have completed their tasks. Thus the looser relationshipallows each state machine to advance as far as possible in each cycle,until it reaches a point it needs another signal to proceed. Thisinterdependent nature therefore allows very efficient handling ofpipelined conditions.

The memory controller is further logically reorganized from aconventional case. The system memory is not located off an intermediateor host bus, but is located directly off the processor bus. The host busis also located off the processor bus. Thus the three buses form a threepoint connection. A data buffer is located at this three pointconnection and is capable of handling data simultaneously on its threebuses. The number of potential data routes is thereby increased over theconventional case, where all processor-memory operations need the use ofthe host bus, interfering with processor-host bus operations. With datacapable of going on two separate paths, opportunities for concurrencyare increased.

The memory controller is logically organized as three main blocks, onecalled the front end being connected to and responsible for processorbus interaction. A memory block is provided to handle interaction withthe actual memory devices and communicates with the front end block toindicate cycle start and completion. Similarly, a host block is providedto handle host bus interaction and also communicates triggering eventsto the front end block.

The front end block decodes a cycle, determines whether it goes to thememory or the host bus and initiates the proper block. The front endblock is then free to work on the next cycle, so pipelining is readilyhandled. If the next cycle is intended for the other of the memory orthe host bus, that block can be activated and it begins its operations.Now full concurrency is developed. The front end block is free to decodeyet another cycle, assuming one has been provided by the processor. Thuseven more pipelining can be developed. In the preferred embodiment anextended industry standard architecture (EISA) bus is developed from thehost bus. As write operations can be posted from the host bus to theEISA bus, it is apparent that even more concurrency can be obtained ifan EISA-intended write operation is interposed.

The arrangement of the memory controller as a plurality of statemachines simplifies this concurrency. Fewer states are needed in totalas the possibilities are handled individually rather than as a group.Thus the overall complexity of the memory controller is greatly reduced,down to a manageable size, yet pipelining and concurrency are obtainedand maximized.

In addition to the different logical arrangement, the memory controlleris further segmented in to a different physical arrangement. The addressmultiplexing, MRAS and MCAS signal qualification and MWE signal transferare all performed on a single chip or device to minimize interdeviceconcerns. Further, the signals are all provided synchronously fromflip-flops clocked off the same signal, resolving the asynchronousversus synchronous problems. As a result, in the preferred embodiment afull clock cycle is saved on every memory cycle as compared to theconventional prior art arrangement.

The flexibility provided by the plurality of state machines also allowsthe memory controller to utilize differing speed memory devices, such as60 ns and 80 ns, on an individual basis, with the memory controllerrunning each memory device at its full designed rate, not the lowest ofthe rates in the memory system. The speed of the memory is stored foreach 128 kbyte block of memory. This stored value is then used when thememory cycle is occurring to redirect a state machine, thusaccomplishing a timing change of the memory devices. As this value isavailable for each 128 kbyte block, each memory device can be operatedat its full speed.

Therefore a memory controller according to the present invention makesmaximum use of any address pipelining provided by the processor, can runnumerous cycles concurrently, dynamically adjusts its operation fordiffering speed memory devices and has reduced skew and propagationdelay. This is done without extreme complexity, so that reliableoperations under all possible operating conditions can be more easilydetermined.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the present invention can be obtained when thefollowing detailed description of the preferred embodiment is consideredin conjunction with the following drawings, in which:

FIG. 1 is a block diagram of a computer system incorporating the presentinvention;

FIG. 2 is a block diagram of the system board of FIG. 1;

FIG. 3 is a block diagram of the input/output board of FIG. 1;

FIG. 4 is a block diagram of the processor board of FIG. 1;

FIG. 5 is a more detailed block diagram of portions of the processorboard of FIG. 4;

FIG. 6 is block diagram of the address/control buffer of FIG. 4;

FIGS. 7, 8 and 9 are block diagrams of the data buffer of FIG. 4;

FIG. 10 is a block diagram of the major portions of the memorycontroller of FIG. 4;

FIGS. 11-22, 24-34, 37-39, 42 and 44-50 are state machine diagrams for anumber of the state machines in the memory controller of FIG. 4;

FIGS. 23, 35, 36, 38A-38I, 40, 41, 43 and 51 are schematic diagrams ofportions of the memory controller of FIG. 4 used with various statemachines; and

FIGS. 52-69 are timing diagrams of various cycles of the memorycontroller of FIG. 4;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a computer system C according to the preferredembodiment is shown. A processor board P contains the processor, cachememory and main memory and associated equipment and a connector 100,preferably a card edge. A system board S includes a connector 102 toreceive the connector 100, preferably a socket to receive a card edge.The system board S contains common system elements and slots orconnectors for interchangeable circuit boards. The system board S alsocontains an additional connector 104. The connector 104 mates with aconnector 106 on an input/output (I/O) board I. Preferably the I/O boardI contains certain I/O related features of the computer C, such as thefloppy and hard disk drive control units, an audio system and theparallel and serial ports. Additionally, the real time clock andcomplimentary metal oxide semiconductor (CMOS) memory is on the I/Oboard I. Each of the processor board P, system board S and I/O board Iare detailed below.

This is noted to be an exemplary and preferred embodiment of thecomputer system C and it is understood that numerous other embodiments,such as having all of the components on a single system board or motherboard as is common, could be readily developed.

Referring now to FIG. 2, a block diagram of the system board S is shown.Two main buses, a host bus H and an EISA bus E, form the logical backbones of the system board S. The host bus H has three components, thehost data or HD bus, the HA or host address bus and the HC or hostcontrol bus. Preferably the HD bus is 32 bits wide and the HA bus isprovided for a 32 bit addressing space. Preferably the host bus Hoperates substantially according to an 80486 protocol, but includingcapabilities for pipelining as in the 80386. The EISA bus E has fourmajor components, the SA and LA or system and early address buses, theSD or system data bus and the SC or system control bus. A plurality ofEISA slots 108 are connected to the EISA bus E. An EISA bus controller110 such as the Intel 82358, provides the necessary capabilities toconvert between host bus H cycles and EISA bus E cycles and is connectedbetween the HA and HC buses and the SC bus. The EISA bus controller 110is connected to control a data EBB or EISA bus buffer 112, whichprovides the necessary data transceiving functions between the host busH and the EISA bus E and provides the data assembly and disassemblyrequirements of an EISA system. Similarly, the EISA bus controller 110provides the control function for an address EISA bus buffer or EBB 114.The address EBB 114 provides transceiving capabilities between the EISAbus E and the host bus H and in addition latches the HA bus signals toform the SA bus signals as appropriate.

The computer C includes certain fundamental components such as aninterrupt system, a direct memory access (DMA) controller and a numberof timers, as well as arbitration of the EISA bus E. These componentsare all contained in the EISA system peripheral 116, which is connectedto the HA, HC and SC buses. A chip referred to as the system glue chip118 is connected to the EISA bus E and to a bus referred to as the XD orX data bus and performs numerous miscellaneous functions necessary inthe computer system C.

Finally, a third bus, referred to as the X bus X, is provided to theconnector 104. The X bus X has address XA, data XD and control SCportions. The XA lines are developed from the SA bus by means of abuffer 120, while the XD bus is formed from the SD bus by means of abuffer or transceiver 122. Preferably the XD bus is 16 bits wide, whilethe XA bus is the same width as the SA bus. The host bus H and the XDbus are provided to the connector 102 for provision to the processorboard P. In addition, the XD bus is connected to the EISA systemperipheral 116.

Referring now to FIG. 3, the I/O board I is shown in more detail. Thebackbone of the I/O board I is the X bus X. Connected to the X bus X isa need only memory (ROM) or read only memory 124, which contains thebasic operating sequences of the computer system C. The ROM 124 ispreferably a flash erasable programmable read only memory (EPROM) toallow ease of changing of the basic operating sequences. A real timeclock (RTC)/CMOS unit 126 is connected to the X bus X to provide realtime clock functions and longer term memory capabilities. A floppy diskcontroller 128 is connected to the X bus X and receives a floppy diskunit 130. A keyboard controller 132, typically an 8042 device, isconnected to the X bus X and receives a keyboard 134 and a mouse orpointing device 136. A multiple peripheral chip 138 is connected to theX bus X and provides an interface to a hard disk unit 140, a parallelport 142 and two serial ports 144 and 146. Finally, an audio system 148is connected to the X bus X to provide audio functions, with a speaker150 connected to the audio system. Other components could be provided onthe I/O board I if desired. Preferably, the graphics system in thepreferred computer system C is provided on a separate add in cardlocated in an EISA slot 108 and is not necessarily located on the I/Oboard I.

Referring now to FIG. 4, the processor board P is shown in more detail.The primary component on the processor board P is the processor or CPU152, in the preferred embodiment preferably the P5 or Pentiummicroprocessor from Intel Corp., the next generation design after the80486. In the preferred embodiment the 66 MHz version is utilized.Operation and interfacing of the 80386 and 80486 are assumed to befamiliar to the reader. The P5 is a very high performance microprocessorhaving a superscalar architecture and integrated and separate code anddata caches. The data bus is 64 bits wide and 8 parity bits areprovided. The data cache is a write-back design having a 64 byte linewidth. Many of the signals of the P5 have a similar function as in the80386 and 80486 and only those signals have a changed or new functionwill be described in detail herein. One signal is CACHE, which is usedto indicate whether the cycle is cacheable by the processor 152. Anadditional signal used to indicate cacheability is the PCD or page cachedisable signal, which is used to disable caching of selected pages ofmemory. If the CACHE signal is present and the PCD signal is notasserted during a read or write cycle, they are burst cycles, namelycache line fill or write-back. Further, the P5 has only a PBRDY* orprocessor burst ready input and does not have a PRDY* or single cycleready input, so the PBRDY* input is used for all ready indications tothe processor 152. The P5 does include a PNA* or next address input toallow address pipelining. One additional feature in the P5 is SystemManagement Mode or SMM. This is similar to that in the Intel 80386SL and80486SL processors. An SMI pin is used to enter SM mode and a signalreferred to as SMIACT* is provided to indicate operation in SM mode.

Three buses are connected to the P5 CPU 152, the PD or processor databus, the PA or processor address bus and the PC or processor controlbus. A level two (L2) or external cache 154 is connected to the PD andPA buses and receives control signals from a memory controller 156. Inthe preferred embodiment the memory controller 156 is a singleapplication specific integrated circuit (ASIC) which contains the memorycontroller functions and additionally includes the cache controllercapabilities necessary to operate the L2 cache 154.

A data buffer 158 is connected to the PD bus and develops two new buses,the HD or host data bus and the MD or memory data bus. The HD bus isconnected to the connector 100 for connection to the system board S. Thedata buffer 158 is controlled by the memory controller 156. Atransceiver/latch unit 160 is connected between the PA bus and the HAbus to provide latching and transceiving capabilities of addressesbetween the P5 CPU 152 and the host bus H. The transceiver/latch 160 iscontrolled by the memory controller 156 to allow posting of cycles fromthe processor bus to the host bus H.

A processor utility chip 162 provides certain necessary utilityoperations for use with the CPU 152. The processor utility chip 162 isconnected to the XD bus and the HC bus and is controlled by the memorycontroller 156. The output of the processor utility chip 162 ispreferably provided to the PC bus to provide control functions of the P5CPU 152.

The memory controller 156 is also connected to a unit referred to as theDDF or data destination facility 164. The DDF 164 performs memory moduleenabling, address translation and memory segment or page propertystorage. The DDF 164 provides memory capabilities to indicate whichparticular bank of memory is to be enabled, performs address translationon a 128 kbyte boundary and provides indications of certaincharacteristics of each 128 k block, such as whether it is located inthe main memory on the processor board P or is external, writeprotected, high speed, and/or cacheable and whether that portion can beutilized by a write-back cache. Operation of the DDF 164 is morecompletely explained in U.S. Pat. No. 5,341,494 issued to Thayer, et al.and hereby incorporated by reference. The outputs of the DDF 164 are aseries of RASEN or RAS enable signals for enabling a particular memorymodule, certain translated addresses to indicate the address bits above128 k, and the page characteristic bits, such as HNCA, HCW, HWP,PLOCMEM*, and SPD60.

The memory portion of the processor board P is provided as fouridentical modules, each module containing an address/control buffer 166,one socket for receiving an individual single in-line memory module(SIMM) unit 168 and base memory 170. The address/control buffer 166receives the PA bus, the address and enable outputs of the DDF 164 andcontrol signals from the memory controller 156. The outputs of theaddress/control buffer 166 are the addresses provided to the SIMMs 168or base memory devices 170 and the RAS*, CAS* and WE* signals. Thesesignals are provided to each of the SIMMs 168 and the base memorydevices 170. Additionally, each SIMM 168 or base memory device 170 isconnected to the memory data bus MD. As indicated, there are four likemodules. Other configurations of the processor board P could bedeveloped, with variations obvious to one skilled in the art, withcertain details provided below.

Referring now to FIG. 5, the various control signals utilized by orprovided by the memory controller 156 are shown, with their appropriateconnections to the related bus or block. FIG. 5 is useful for anoverview of the connections of the memory controller 156 to theremaining components of the computer system C.

Referring now to FIG. 6, a block diagram of the address/control buffer166 is shown. The address/control buffer 166 is made as a single chip,for reasons discussed below. The MSELCA* or memory select column addresssignal is provided to a D-type flip-flop 200. The clock of the flip-flop200 is provided by the PCLK signal, which is preferably the 66 MHzsignal used as the clock by the processor 152. The noninverted output ofthe flip-flop 200 is provided to the select input of a 12 bit 2:1multiplexor 202. The MALE or memory address latch enable signal isprovided to the D input of a D-type flip-flop 204, the flip-flop beingclocked by the PCLK signal. The noninverted output of the flip-flop 204is the SMALE signal and is provided to the enable input of a 21 bitlatch 206. The D inputs of the latch 206 receive the PA<16 . . 4>signals from the PA bus and the TA<25 . . 17> translated address signalsfrom the DDF 164. The noninverted outputs of the portions of the latch206 receiving signals PA<16 . . 13> and TA<24, 22 . . 17> are providedto the one input of the multiplexor 202. The noninverted outputs of theportions of the latch 206 receiving signals PA<12 . . 5> and TA<25,23>are provided as inputs to the zero input of the multiplexor 202. Thenoninverted output of the latch 206 portion receiving the PA<4> signalis provided to the address input of a burst logic block 210. The burstlogic block 210 is used to increment, effectively invert, the addressbeing applied to the memories during burst operations, that is, cacheline fill and writeback operations, of the processor 152. The burstlogic 210 also receives the MALE signal to indicate when to load anaddress and the MADRINC* signal to increment the address when active.Further, the PCLK signal is provided to the burst logic 210 to performthe actual clocking operation. The output of the burst logic 210 is theleast significant bit of the column addresses and is provided to thezero input of the multiplexor 202. The output of the multiplexor 202 isprovided to the D input of a 12 bit D-type flip-flop 212. The PCLKsignal clocks the flip-flop 212 and the Q or noninverted outputs are theMA<11 . . 0> signals.

The REFRESH* signal is provided to the D input of a latch 214 whoseenable input receives the SMALE signal. The output of the latch 214 isprovided as one input to a RAS decode logic block 216. The RASEN<1 . .0> signals from the DDF 164, which indicate which bank of memory is tobe activated, either the base memory 170 or the SIMMs 168, are providedto the D inputs of a two bit latch 218. The enable input to the latch218 is the SMALE signal. The output of the latch 218 is provided as theother inputs to the RAS decode logic 216. A further input to the RASdecode logic 216 is the LMRAS* signal described below. The output of theRAS decode logic 216 is a four bit signal which is active to indicatethat either a proper RAS has been selected or in the case of REFRESHoperation that all row addresses needed to be provided. The output ofthe RAS decode logic 216 is provided to a four bit latch 220 whoseenable input receives the LMRAS* signal which is provided by thenoninverted output of a D-type flip-flop 222. The MRAS* signal isprovided to the D input of the flip-flop 222, which is clocked by thePCLK signal. In this case the master RAS or MRAS* signal is thus used toenable the decoded RAS signals. The two noninverted outputs of the latch220 related to a first set of decoded RAS signals are provided to oneinput of two OR gates 224 and the other two noninverted outputs of thelatch 220 related to a second set of decoded RAS signals are provided toone input of two OR gates 226. The second input to the OR gates 224 and226 is provided by the LMRAS* signal. The outputs of the OR gates 224are provided to the D inputs of a two bit D-type flip-flop 228, which isclocked by the PCLK signal and whose output are the RASA*<1 . . 0>signals. The outputs of the OR gates 226 are provided to the D inputs oftwo bit D-type flip-flop 230, which is clocked by the PCLK signal andwhose outputs are the RASB*<1 . . 0> signals. The RASEN<1 . . 0> signalsto RASA*<1 . . 0> and RASB*<1 . . 0> signals decoding is shown below,with memory banks 0 and 1 corresponding to the base memory 170 and banks2 and 3 corresponding to the SIMMs 168.

    __________________________________________________________________________    RASEN<1 . . . 0>                                                                        RASA*<1>                                                                             RASA*<0>                                                                             RASB*<1>                                                                             RASB*<0>                                                                             Bank                                    __________________________________________________________________________    11        1      0      1      1      0                                       10        0      1      1      1      1                                       01        1      1      1      0      2                                       00        1      1      0      1      3                                       __________________________________________________________________________

Four of the processor byte enable signals, that is PBE*<3 . . 0> orPBE<7 . . 4> depending upon which address/control buffer 166 is beingreferenced, each address/control buffer 166 handling 32 bits of memory,are provided to the D inputs of a 4 bit latch 232. The enable signal forthe latch 232 is the SMALE signal. The outputs of the latch 232 are fourinputs to CAS decode logic 234. The PA<3> signal is provided to the Dinput of a latch 236, which is enabled by the SMALE signal. The outputof the latch 236 is provided to the CAS decode logic 34. The MALLBES*memory or all byte enables signal from the memory controller 156 isprovided to the D input of a latch 238, which is enabled by the SMALEsignal. The output of the latch 238 is provided to the CAS decode logic.The CAS decode logic 234 has four outputs, namely the four lines whichare to activate the particular bytes of memory to be provided by thisaddress control buffer 166. The PA<3> signal is used to do selectionbetween the upper and lower 64 bits of the memory system and the PBE*signals are used for specific byte lane encoding of the 32 bits. In thepreferred embodiment, the memory is 128 bits wide but eachaddress/control buffer 166 only controls 32 bits, so two address/controlbuffer chips each receive the same PBE, signals. To resolve interleavingproblems, an input (not shown) is used to indicate whether theparticular address/control buffer 166 is for the upper or lower 64 bits.This signal is compared with the PA<3> signal for effective chipselection to prevent aliasing problems. The four outputs of the CASdecode logic 234 are provided to the D input of a 4 bit latch 240. Theenable signal to the latch 240 is the LMCAS* or latched master CASsignal which is provided by the noninverted output of a D-type flip-flop242. The D input of the flip-flop 242 receives the master CAS or MCAS*signal and is clocked by the PCLK signal. The four outputs of the latch240 are provided as inputs to four two input OR gates 244 and as inputsto four two input OR gates 246. The other inputs to the OR gates 244 and246 are provided by the LMCAS* signal. The outputs of the OR gates 244are provided to a 4 bit D-type flip-flop 248 which is clocked by thePCLK signal and whose noninverted outputs are the CASA*<3 . . 0>signals. Similarly, the four outputs of the OR gates 246 are provided tothe D-type inputs of a 4 bit D-type flip-flop 250 which is clocked bythe PCLK signal and whose noninverted outputs provide the CASB*<3 . . 0>signals. The CASA*<3 . . 0> signals are provided to the base memory 170,while the CASB*<3 . . 0> signals are provided to the SIMMs 168.

The MWE* signal is provided to the D input of a D-type flip-flop 252,which is clocked by the PCLK signal. The noninverted output of theflip-flop 252 is provided to the D input of D-type flip-flops 254 and256. The flip-flops 254 and 256 are clocked by the PCLK signal andproduce, respectively, the MWEA* and MWEB* signals, which are the memorywrite enable signals provided to the base memory 170 and the SIMMs 168,respectively.

Thus by properly providing the MRAS*, MCAS*, MALE, MALLBES*, MADRINC*and MWE* and MSELCA* signals to the address/control buffer 166, thememory controller is able to properly provide the RAS, CAS and writeenable signals in conjunction with the proper memory addresses to allowpage mode operation as well as interleaved operation.

Thus it can be seen that all of the signals necessary to control thememory devices are provided to the memory devices from theaddress/control buffers 166. As the address/control buffers 166 areformed from a single chip, the skew between signals is greatly reducedfrom the conventional case where the signals are provided from multiplechips, each potentially having different delays even if of the sameintegrated circuit technology. Secondly, as the signals are all outputsynchronously from the output flip-flops, skew between the paths takenby the signals is also greatly reduced versus the conventional casewhich includes both synchronous and asynchronous signal development.

In the system of the preferred embodiment, it has been determined that afull cycle of the 66 MHz clock can be saved for every memory cycle inthis manner. The chart below indicates the timing margin calculationsfor a system according to the prior art, such as the one disclosed inBonella, U.S. Pat. No. 5,333,293 and hereby incorporated by reference,and a system according to the preferred embodiment. The example is forthe row address hold (RAH) from RAS timing parameter for the preferred60 ns DRAMs, but similar examples can be developed for the other morecritical timing parameters.

    ______________________________________                                                             MIN    MAX                                               ______________________________________                                        Prior Art Example                                                             Budget - 2 Clock cycles    30ns     30ns                                      Memory Controller Clock    -4.9ns   -12.7ns                                   to MRAS valid                                                                 MRAS derating to RAS                                                                             max     -2ns     -2ns                                      buffers                                                                       MRAS to RAS (prop delay)                                                                         max     -8ns     -8ns                                      RAS derating       max     -2ns     -2ns                                      Memory Controller Clock    +5.5ns   +14.1ns                                   to MA valid                                                                   Memory controller MA                                                                             min     +0.5ns   +0.5ns                                    derating                                                                      MA buffer delay    min     +2ns     +2ns                                      MA derating        min     +0.5ns   +0.5ns                                    t.sub.RAH - DRAM Row Address                                                                     min     -12ns    -12ns                                     Hold                                                                          Margin                     6.6ns    8.4ns                                     Preferred Embodiment Example                                                  Budget - 1 Clock cycle     15ns     15ns                                      Address/control buffer     -2.8ns   -8.9ns                                    clock to RAS valid                                                            RAS derating       max     -2ns     -2ns                                      Address/control buffer     +3.2ns   +12.9ns                                   clock to MA valid                                                             MA derating        min     +0.5ns   +0.5ns                                    t.sub.RAH - DRAM Row Address                                                                     min     -12ns    -12ns                                     Hold                                                                          Margin                     1.90ns   5.5ns                                     ______________________________________                                    

MIN refers to minimum timing, when the intergrated circuit is made usingits fastest process, is at its lowest low temperature and is at itshighest voltage. MAX refers to maximum timing, when the integratedcircuit is made using its slowest process, is at its highest temperatureand is at its lowest voltage.

While in the preferred embodiment the address multiplexor 202 is locatedin the address/control buffer 166, it is understood that the actualmultiplexor could be provided external to the address/control buffer166, but the multiplexed form of the addresses would still be providedto the address/control buffer 166 to allow driving and synchronizationby a single chip. The presence of the multiplexor 202 on theaddress/control buffer 166, along with the address latches 206 and 236and byte enable latches 232 is preferred as this variation reduces pincounts of other chips and more easily allows operation in a pipelinedformat.

FIGS. 7, 8 and 9 disclose more details on various portions of the databuffer 158. Referring now to FIG. 7, the PD bus bits 63 to 0 and theparity bits associated with these lines are provided to the inputs of a72 bit buffer 270. The outputs of the buffer 270 are provided to the Dinputs of four 72 bit latches 272, 274, 276 and 278. These four latchesreceive as enable inputs the LPDMDEN0, LPDMDEN1, LPDMDEN2 and LPDMDEN3signals. The use of four latches 272-278 allows an entire burst from theprocessor 152 to be stored in a write posting operation. The outputs ofthe latch 272 go to the zero input of a 3:1 72 bit multiplexor 280. Theoutputs of the latch 276 go to the one input of the multiplexor 280. Theoutputs of the latch 274 go to the zero input of a second 72 bit 3:1multiplexor 286. The outputs of the latch 278 similarly go to the oneinput of the multiplexor 286. The select inputs to the multiplexors 280and 286 are connected to the LMDSEL<1 . . 0> signals. The outputs of themultiplexors 280 and 286 are combined in parallel to produce a 144 bitwide path which is provided to the input of a 144 bit tri-state buffer288. The output control for the buffer 288 is inverted and is connectedto the LMDOE* signal. The output of buffer 288 is the memory data bus,bits 127 to 0 of memory data and 16 bits of parity.

A 36 bit latch 290 and a second 36 bit latch 292 are connected inparallel to the output of the buffer 270. The enable inputs to thelatches 290 and 292 are provided by the LPDHDEN or latched processordata to host data enable signal. The output of the latch 290 is providedto the zero input of a 32 bit 3:1 multiplexor 296. Parity data is notprovided to multiplexor 296. The outputs of the latch 292 are providedto the one input of the multiplexor 296. The select input to themultiplexor 296 receives the HDSEL<1 . . 0> signals or host data selectsignals. The outputs of the multiplexor 296 are provided to a 32 bittri-state buffer 298 whose inverted control input receives the HDOE*signal. The output of the buffer 298 is connected to the 32 bits of theHD bus.

A channel is also necessary to go from the host data bus to the memorydata bus. To this end a 32 bit buffer 300 is connected to the HD bus andhas its outputs connected to the D inputs of a 32 bit latch 302. TheLMDHDEN or latched memory data-host data enable signal is provided tothe enable input of the latch 302. The outputs of the latch 302 areprovided to a parity generator 304 which provides a 4 bit parity output.The outputs of the latch 302 are also provided to four 32 bit buffers306, 308, 310 and 312. Similarly, the outputs of the parity generator304 are provided to four 4 bit buffers 314, 316, 318 and 320. Theoutputs of the buffers 306, 308, 314 and 316 are paralleled to provideinputs to the third channel of the multiplexor 280 while the outputs ofthe buffers 310, 312, 318 and 320 are paralleled to provide the inputsto the third channel of the multiplexor 286. Thus, it is apparent thatthe same data from the host data bus is provided four times across tothe memory data bus, but this is acceptable because the RAS decode logic216 and the CAS decode logic 234 in the address control buffer 166actually control which data bytes and which memory devices are actuallyactive and receive the data.

It is also necessary to get data from the host data bus to the processordata bus. To this end the output of the buffer 300 is provided to the Dinputs of two 32 bit latches 322 and 324. The LHDPDEN1 signal is theenable input to the latch 322 and the LHDPDEN0 signal is the enableinput to the latch 324. Parity generators 326 and 328 are connected tothe outputs of the latches 322 and 324 respectively. The outputs of theparity generator 326, the latch 322, the parity generator 328, the latch324 are provided to appropriately sized buffers 332, 330, 336 and 334 sothat a 72 bit wide value of 64 data bits and 8 parity bits is providedto the third input of a 72 bit 3:1 multiplexor 338. The selectioncontrol for the multiplexor 338 is provided by the LPDSEL<1 . . 0> orlatched processor data select signals. The output of the multiplexor 338is provided to the inputs of a 72 bit tri-state buffer 340 whoseinverted tri-state control receives the LPDOE* signal. The output of thebuffer 340 is connected to the 72 bits of the processor data and paritybus. By use of the two latches 322 and 324, two complete accesses on thehost data bus can be combined in parallel to provide a 72 bit word whichis provided to the processor.

A path must also be provided from the memory data bus to the processordata bus. To this end a 144 bit wide buffer 342 is connected to receivethe memory data bus and the memory parity values. The outputs of thebuffer 342 are provided to 144 bit latch 344 whose enable input receivesthe LMDPDEN signal. The outputs of the latch 344 are provided to thezero and one inputs of the multiplexor 338, with the lower 64 bits andassociated parity to the zero input and the upper 64 bits and associatedparity to the one input. In this manner the multiplexor 338 can be usedto select between banks or interleaved portions of the memory system.

The data portions of the outputs of the buffer 342 are also provided in32 bit segments to 4 inputs of a 4:1 32 bit multiplexor 350. Theselection control for the multiplexor 350 is provided by the MDHDSEL<1 .. 0> signal, which indicates which 32 bit portion of the memory data isto be provided to the host data bus. The output of the multiplexor 350is provided to a 32 bit latch 352, whose enable input receives theLMDHDEN signal. The output of the latch 352 is connected to the thirdinput of the multiplexor 296.

As can be seen, the data buffer 158 contains the necessary linkages andconnections to allow data to be transferred between any two of the busesin either direction. Additionally, the buffer 158 contains sufficientlatching to allow an entire burst write operation from the processor tobe posted. Similarly, the multiple operations from the host bus can beposted or gathered until assembled for provision to the processor systemif necessary. Further, a write operation from the processor to the hostbus can be posted.

Referring now to FIG. 8, the latching logic is shown for the processordata to host or memory data bus signals. The PDHDLE* signal is providedto a buffer 360, whose output provided to the D inputs of a latch 362and of a D-type flip-flop 364. The PCLK signal is provided to theinverted enable input of the latch 362 and to the clock input of aflip-flop 364. The inverted outputs of the latch 362 and flip-flop 364are provided as the two inputs to a two input AND gate 366, whose outputis the LPDHDEN signal. Similar arrangements of buffers, latches,flip-flops and AND gates are utilized with the PDMDLE*<3 . . 0> signalsto produce the LPDMDEN<3 . . 0> signals.

Referring now to FIG. 9, the PDOE* signal is provided to a buffer 368whose output is provided to the D input of a D-type flip-flop 370. ThePCLK signal clocks the flip-flop 370. The noninverted output of theflip-flop 370 is the LPDOE* signal. Again, similar buffer and flip-floparrangements are used with the PDSEL<1 . . 0>, HDPDLE<1 . . 0>, MDHDLE,MDPDLE, MDSEL<1 . . 0> and MDOE* signals to produce respectively, theLPDSEL<1 . . 0>, LHDPDEN<1 . . 0>, LMDHDEN, LMDPDEN, LMDSEL<1 . . 0> andLMDOE* signals.

Referring now to FIG. 10, portions of the memory controller 156 areshown. Preferably the memory controller 156 is organized as a series ofinterdependent blocks. Each of these blocks is logically organized torelate to a particular external unit, such as the L2 cache 154, theprocessor 152, the memory, the host bus H or arbitration, to which it isrelated. A first block is referred to as FECON or the front endcontroller 400. The FECON block 400 interacts with the processor 152 forreceipt of cycle start signals from the processor 152 and providessignals which activate the MCON block 402 and the HCON block 404.Similarly, the FECON block 400 provides completion signals to theprocessor 152 when appropriate upon receipt of handshake signals fromthe MCON block 402 and the HCON block 404. The FECON block 400 alsoprovides signals to the data buffer 158 to latch processor write dataand enable processor read data. The MCON block 402 is the second majorblock and is the memory controller portion. The MCON block 402interfaces with the address/control buffers 166 to provide the necessarycontrol signals and the data buffer 158 to latch data from the memoryand enable data to the memory. The HCON block 404 is the host buscontrol block and interfaces with the host bus H to provide thenecessary signals for its operation. The HCON block 404 furthercommunicates with the data buffer 158 to latch data from the host bus Hand enable data to the host bus H. Both the MCON block 402 and the HCONblock 404 have handshake communications with the FECON block 400 as willbe more apparent in the description below.

A CCON block 406 is the L2 cache controller and interfaces with theFECON block 400 and the cache 154 to provide complete cache controllerfunctions. Preferably the cache controller is a 256 kbyte, directmapped, write through, lookaside cache for simplicity of logic andoperations. A block 408 referred to as HARB or host arbitration blockinterfaces with the FECON block 400 to provide processor and host busarbitration. The final block shown in FIG. 10 is the MPMON block 410 ormemory performance monitor. In memory controller 156 of the preferredembodiment the memory performance monitor 410 is utilized to determinethe state of the MRAS* signal at the end of a memory cycle. Effectivelythe MPMON block 410 is performing next operation prediction so that theMRAS* signal can be properly held low or raised high, depending uponwhether the next operation has a probability of being a memory page hitor memory page miss. More of this will be described with the statemachines described below.

Another block not shown in FIG. 10 for simplicity is the DDFCON or datadestination facility control block. This is omitted for simplicity butprovides the necessary and conventional signals to operate with the DDF164 to allow it to be programmed and read. This operation will beapparent based on the previously referenced patent applications and theoperation of the FECON 400.

Preferably the FECON block 400, MCON block 402 and HCON block 404 arecomprised of a plurality of state machines, each acting independentlyand interdependently, and certain loose logic. Each state machinereceives certain signals to commence its operation and proceedsaccording to the receipt of other signals. The various tasks necessaryfor the memory controller operation are split between these numerousstate machines and between the control blocks as indicated, to allowpipelining and multiple concurrent cycles without requiring an overlycomplex single master state machine. By the use of the plurality ofindividual state machines each operation is allowed to proceed at itsgreatest possible rate, stopping only when another signal or informationis necessary to allow it to proceed. This arrangement and operation willbecome apparent as the numerous state machines are described below andby reference to include timing diagrams. A number of these signalsprovided and utilized internal to the memory controller 156 and betweenthe various control blocks are shown in FIG. 10. The listing is notnecessarily complete but shows the more important signals. Furtherdetails of the CCON or cache controller 406 will not be provided as itis not particularly relevant to this operation except as necessary whencertain cache operations are occurring.

Referring now to FIG. 11, the FECON block 400 itself is made up of aseries of sub-blocks or modules. The first block is the PCM or processorcontrol module 410. This module 410 primarily receives the statussignals from the processor 152 and starts operation of a cycle. A secondblock is the MCM or memory control module 412, which interfaces moredirectly with the MCON block 402. An HCM block 414 is used to interfacewith the HCON block 404. A block referred to as PNAGEN or processor nextaddress generation 416 is utilized to provide the next address or NA*signal to the processor 152 to allow full use of address pipelining. Ablock 418 referred to as PBRDYGEN or processor burst ready generation isused to provide the ready signal to the processor 152 to indicate thatthe data portion of a cycle is completed and the next data portion canproceed. The final block is the DPSELCM block 420 which is used toselect the data path from the MD bus or the HD bus to the PD bus.

Referring to FIG. 12, the MCON block 402 is shown with its constituentsub-blocks. A block 430 is a CPUMEM block which effectively interfacesthe processor 152 with the memory itself. Included in the CPUMEM block430 is the processor memory state machine which is utilized to developthe RAS and CAS and other signals for processor cycles. Unless otherwiseindicated, all state machines advance on rising edges of the PCLK orprocessor clock signal. A block 432 referred to as the ISABM or ISA busmaster block is used to interface with ISA bus masters and it containsthe memory state machine used with ISA bus masters. Similarly, a block434 referred to as EISABM or EISA bus master provides the state machineand memory control when an EISA bus master is in control and isrequesting memory operations. A block 436 referred to as the MABCON ormemory address bus control block is utilized to handle the memoryaddressing related signals. A MDBCON or memory data bus controller block438 is used to handle the latch enable and output enable signals of thedata buffer 158 for memory and host transfers. A block 440 referred toas SNPCON or snoop control is used to indicate to the level 2 cache 154and the processor 152 when snooping of a processor bus operation isappropriate to allow proper cache coherency and write-back operation.The final block is the REFSM or refresh state machine block 442 whichcontrols refresh operations to the memory devices.

FIG. 13 is yet another similar block breakdown for the HCON block 404.The HSYNC or host synchronization sub-block 446 is used to track thehost bus H to determine cycle operations. The HOST or host state machineblock 448 is a host bus master used to drive operations from theprocessor 152 to the host bus H. A block 452 referred to as HACON orhost address control handles address related signals, while a block 454referred to as HDCON or host data control controls the data operations.It is understood that each of the blocks 400, 402 and 404 include othermiscellaneous logic besides those sub-blocks shown.

Proceeding now to FIG. 14, the first state machine in the PCM block 410is shown. As previously noted, unless otherwise indicated, all of thestate machines described herein advance on rising edges of the PCLKsignal. This is the PPROG or processor cycle in progress state machineand is used for basic cycle tracking of the processor bus and providestwo primary signals, the PPROG or processor cycle in progress in thememory controller 156 signal and the PEOC or processor end of cyclesignal. Variations on these particular signals are provided for variousmodules. On reset, the PPROG state machine starts at state A, the idlestate. The PPROG state machine proceeds from state A to state B whenread operations, as indicated by the state of the PWR or processorwrite/read signal when a processor cycle is pending, as indicated by thePPEND signal as it has been pipelined, or upon receipt of the DSPADS ordelayed synchronized processor ADS signal. The SPADS signal is the ADS*signal from the processor 152 which has been synchronized to the PCLKsignal and inverted. The DSPADS signal is the SPADS signal delayed onePCLK signal cycle. Under these conditions control proceeds to state B.If the processor 152 is not performing a burst cycle in state B, thePEOC signal is provided during that state. Control proceeds from state Bto state C while burst operations are occurring, as indicated by thePBURST signal, and when the previous data cycle is completed, asindicated by the PBRDY signal. Control proceeds from state B to state Fwhen a cycle is pending, it is not a burst, the previous cycle hascompleted and the cycle that is pending is a valid write cycle, that is,it is not a memory code write. Control returns from state B to state Aif no further cycles are pending, if a burst operation is not occurringand after the data transfer has been completed as indicated by the PBRDYsignal, thus indicating an idle condition. In all other cases,particularly when the PBRDY signal has not been asserted, controlremains at state B.

Control proceeds from state C to state D when the PBRDY signal isasserted to indicate that a data operation has been completed andotherwise remains in state C. Control proceeds from state D to state Ewhen the next data operation has been completion as indicated by thePBRDY signal. Otherwise control remains at state D. There are threepossible exits from state E and a looping at state E. The PEOC signal isasserted in state E. If the final data value of the burst has beenindicated as being transmitted, another cycle is pending, and it is aread operation which is pending, control proceeds from state E back tostate B and the next cycle proceeds. If the data value is indicated asbeing provided and a cycle is not pending, control returns to state A.If a cycle is pending, it is a valid write cycle and the data transferhas been completed, control proceeds to state F to begin a writeoperation. In all other cases, particularly until the data operation hasbeen completed, control remains at state E.

Control proceeds from state A to state F if a valid write operation isindicated and a cycle is pending or the DSPADS signal is active. Controlremains in state A during all cycles which are not transfers to states Bor F. In state F the PEOC signal is asserted if a burst operation is notoccurring. If a burst operation is not occurring, the data transfer hasbeen completed as indicated by the PBRDY signal and no cycle is pending,control proceeds from state F to state A and idles until the next cycle.If the data has been transferred as indicated by the PBRDY signal and aburst operation is occurring, control proceeds to state G. If a readcycle is pending, a burst is not occurring and the data transfer hasoccurred, control proceeds to state B. Otherwise control remains atstate F, particularly until data is ready. Control proceeds from state Gto state H if the PBRDY signal is provided and otherwise remains instate G. Control proceeds from state H to state I upon receipt of thePBRDY signal and otherwise remains at state H. In state I the PEOCsignal is asserted. Control proceeds from state I to state A when thePBRDY signal is asserted and otherwise remains in state I. In all statesother than state A the PPROG or processor cycle in progress signal isasserted.

As indicated in discussing the PPROG state machine, the signal referredto as PPEND or processor cycle pending is necessary to allow full use ofpipelining. This signal is provided by the PPEND state machine shown inFIG. 15. This is a simple two state state machine which is used toindicate that a processor cycle has been requested while one iscurrently in progress. Upon reset, the PPEND state machine proceeds tostate A, the idle state. Control proceeds from state A to state B if theSPADS or synchronized processor ADS signal is received and the PPROGsignal is true. In state B the PPEND signal is asserted. Controlproceeds from state B back to state A upon receipt of the SEOC signal,which is a PCLK synchronized version of the ANDing of the PBRDY signaland PEOC signal, when the SPADS signal is not asserted. Otherwisecontrol remains in state B.

The PCM module 410 is also responsible for developing the PDOE* signal.This is done in the DPOE state machine as shown in FIG. 16. Controlinitiates at state A upon reset. Control proceeds to state D, a waitstate, if the memory system was currently providing an output asindicated by the DSPADS signal occurring during a read cycle which isnot a memory cache hit when a cycle is in progress (this being theDPOE₋₋ OK signal definition) and the data transfer has not completed asindicated by the PEOC and PBRDY signals. Thus in this mode in state Dthe DPOE state machine is waiting for the PD bus to become ready.Control proceeds from state D to state B upon receipt of the PEOC andPBRDY signals to indicate that the data operation is completing, or fromstate A to state B if this is a non-cache hit memory read cycle and acycle is not currently in progress or if it is a DPOE₋₋ OK condition, acycle is in progress and the end of that cycle is indicated by the PEOCand PBRDY signals. In all other cases control remains at states A and D,as appropriate.

Control proceeds from state B to state A if this is other than a DPOE₋₋OK cycle, an end of cycle has occurred and upon receipt of a signalreferred to as EPBRDY or early processor bus ready, which indicates thatin the next clock cycle the PBRDY signal will be asserted. In this casethe cycle is effectively completed and the output control will be ableto be turned off because as indicated in the data buffer 158, the DPOEsignal which actually controls output is one PCLK signal cycle delayedbecause of the internal synchronizing flip-flop. Control proceeds fromstate B to state C if it is proper to provide an output as indicated bythe DPOE₋₋ OK signal being active and the cycle is not completed or anearly ready has not been received. In all other cases control remains atstate B. Control returns from state C to state B upon receipt of thePEOC and PBRDY signals and otherwise remains at state C. It is notedthat when the next state will be state B or state C or the state machineis in state C, the PDOE* signal is active, that is asserted low. Thus inthis manner for non-cache hit read cycles by the processor 152 theoutput enable is active at the proper time. In general when referring tosignals in this description, an asterisk (*) after a signal mnemonicindicates that it is logically true when a low voltage level is presentand is the inverse of the signal mnemonic without an asterisk and anglebrackets with included numbers after a signal mnemonic are used toindicate single or multiple bit positions in a wider logical signal,such as the data or address fields. Also for this description, statemachine transition conditions are based on the level of the indicatedsignal, not its logical state.

Certain miscellaneous signals are also produced in the PCM module 410.The first of these is the FEHLDA or front end hold acknowledge signal.This is the output of a D-type flip-flop clocked by the PCLK signalwhich has an input of the output of an AND gate which receives as inputssignals indicating none of processor, cache, memory, host bus read andhost bus write cycles are in process and a processor bus hold requesthas been received. The second signal of interest is the PBURST signalwhich is provided by the output of a D-type flip-flop clocked by thePCLK signal and is provided at the beginning of cycles which areprocessor cachable, such as cache line fills and write-backs, or are L2cache 154 read allocations, and is removed at the end of the final oneof the series of such cycles.

Proceeding now to FIG. 17, the first state machine in the memory controlmodule 412 is the MPROG or memory in progress state machine. The statemachine begins at state A upon reset. There are four basic paths out ofstate A, relating to single memory read cycles, burst memory readcycles, single memory write cycles and memory write-back or burst writecycles. Control transfers from state A to state D, which is used forsingle memory read cycles, when DSPADS signal is active, a memory cycleis indicated by the MCYC signal, a L2 cache line fill burst cycle is notin progress as indicated by the FEBURST* signal and a read is indicated.An L2 cache line fill operation is always also a processor 152 cacheline fill as a processor 152 cache line fill miss is the cause of the L2cache 154 line fill. A processor 152 cache line fill supplied by the L2cache 154 is not an operation directed to the main memory and is treatedas any other cache hit. Control returns from state D to state A if theSMRDY or synchronized memory ready signal is provided and a cycle wasnot already in progress on the host bus prior to this memory cycle, asindicated by the MPGHPG₋₋ 1ST signal, and neither a memory cycle ispending nor is another memory cycle starting. The MPGHPG₋₋ 1ST signal isused to delay the return of data so that concurrent read cycles to thehost bus H and memory do not get out of order. Control proceeds fromstate D to state L if the memory is ready and a cache cycle is inprogress or if the SMRDY signal has been received and a host bus cyclewas started first. Control proceeds from state D to state C if thememory indicates ready, a host bus cycle was not already in progress andeither another memory cycle is pending or another memory cycle is beinginitiated. Otherwise control remains at state D.

Control proceeds from state L to state C if a host cycle was not alreadyin progress, a cache cycle is not in progress and either another memorycycle is pending or another memory cycle is commencing. Control proceedsfrom state L to state A if an uncompleted host cycle was not already inprogress, a cache cycle is not in progress and neither a memory cycle ispending nor is another memory cycle commencing. In all other cases,control remains at state L.

If a memory burst read is occurring, control proceeds from state A tostate F as indicated by the DSPADS signal being true, a memory cyclebeing indicated, a L2 cache line fill burst cycle being indicated and aread cycle being indicated. Control proceeds from state F to state E ifeither the memory operation has indicated that it is ready and a cachecycle is in progress or if the ready has previously indicated and a hostbus cycle was in progress first. Otherwise, if the memory control hasindicated that it was ready as indicated by the SMRDY signal and a hostbus cycle was not already in progress, control proceeds from state F tostate G. Otherwise control remains at state F. Control transfers fromstate E to state G when a signal referred to as MRDBHLD₋₋ LV or memoryread burst hold leave is active, indicating completion of the burst. Thedevelopment of the MRDBHLD₋₋ LV signal is described below. Otherwise,control remains at state E.

Control transfers from state G to state C if the memory has indicatedthat it was ready and either a memory cycle is pending or a memory cycleis being indicated as commencing. Control transfers from state G tostate A if the memory has indicated its completion and there is neithera memory cycle pending nor a memory cycle commencing. In other cases,control remains at state G pending the memory completion.

For single memory write cases, control transfers from state A to state Hbased on the DSPADS signal, MCYC and PWR signals being high and anindication that this is not a processor cachable cycle. Control proceedsfrom state H to state C when the memory indicates that it is ready andeither a memory cycle is pending or is commencing. Control proceeds fromstate H to state A if the memory indicates that it is ready and there isneither another memory cycle pending nor a cycle commencing. In allother cases control remains at state H, generally awaiting completion ofthe memory operation.

In the final case for exiting state A, control proceeds from state A tostate I for memory write-back cycles. This is indicated by the DSPADSsignal being true, a memory cycle start is indicated by the MCYC signal,the PCACHE or processor cachable signal is present, and a write isoccurring. Control proceeds from state I to state J when the memoryindicates that it is ready. Otherwise control remains at state I.Control proceeds from state J to state C if the memory is ready andeither another memory cycle is pending or another memory cycle iscommencing. Control proceeds from state J to state A if the memory cycleis completed as indicated and neither another memory cycle is pendingnor commencing. Two readys are necessary in the preferred embodiment inthis case as the processor data cache line size is 256 bits or twomemory operations. Otherwise control remains at state J.

Control proceeds from state C to state D if a memory cycle is indicatedby the MCYC signal, it is not a burst and it is a read or if a signalreferred to as MRDPND or memory read pending is indicated as true. TheMRDPND signal development is described below. Control proceeds fromstate C to state F for read burst cases as indicated by a memory cycleindication, a burst indication and a read or by the presence of thesignal MRDBPND for memory read burst pending. Control proceeds fromstate C to state H on the indication of a memory cycle, the fact that itis not processor cacheable and that it is a write, that is a singlewrite cycle, or the MWRPND signal is active indicating a memory writehas been presented. In all other cases, that is cache write-backs,control proceeds from state C to state I.

The MPROG state machine produces several signals. These signals are asfollows:

MRD=MPROG STATE D

MRDHLD=MPROG STATE L

MRDBHLD=MPROG STATE E

MRDB1=MPROG STATE F

MRDB2=MPROG STATE G

MWR=MPROG STATE H

MWRB1=MPROG STATE I

MWRB2=MPROG STATE J

MPDREQ=(MPROG STATE E+MPROG STATE L)*

MPROG=(MPROG STATE A+MPROG STATE C)*

EMPROG=(MPROG STATE A+MPROG STATE C+MPROG NEXT STATE A+MPROG NEXT STATEC)*

MEMRD=(MCYC & PWR*)+MRD+MRDB1+MRDB2+MRDHLD+MRDBHLD

MEMWR=MWR+MWRB1+MWRB2

Thus it can be seen that the MPROG state machine indicates when memorycycles are present in the memory system by providing the MPROG, MEMRD,MEMWR and other signals.

Referring now to FIG. 18, the MPEND or memory pending state machine isshown. As noted above, the MPROG state machine utilizes as one of itsbranching conditions the fact that a memory cycle is pending. Thissignal is provided by the MPEND state machine. Upon reset the MPENDstate machine proceeds to the idle state or state A. There are fourexits from state A, one for each of single memory read cycle, burstmemory read cycle, single memory write cycle and memory write-back cyclecases. Control proceeds from state A to state C when the DSPADS signal,the MCYC signals and the MPROG or memory in progress signals are trueand a write is indicated and it not a processor cacheable operation.Thus this is the single write case. Control remains in state C while amemory cycle is in progress as indicated by the MPROG state machine.Otherwise control proceeds to state A.

Control proceeds from state A to state D for memory write-back cycles asindicated by the DSPADS, MCYC and MPROG signals being true and a writeindication and an indication that it is processor cacheable. Controlproceeds from state D to state A when a memory cycle is not active andotherwise remains in state D. Control proceeds from state A to state Efor memory cycles which are reads while a memory cycle is in progressand which are not bursts. This is indicated by the DSPADS, MCYC andMPROG signals being true and a read being indicated in a non-burst case.Control proceeds from state E to state A when the memory cycle inprogress has been completed and otherwise remains at state E pendingcompletion. Control proceeds from state A to state F if the DSPADS, MCYCand MPROG signals are true, indicating a memory cycle is in progress andanother one is requested, it is a read cycle and bursting is indicated.Control proceeds from state F to state A when the current memory cyclein progress is completed. Otherwise control remains at state F. TheMPEND or memory cycle pending signal is true when states C or D are tobe entered as the next state or when the state machine is in states C,D, E or F. The MWRPND signal is true when the next state is state C orthe current state is state C. The MWRBPND signal is true when the nextstate is state D or the current state is state D. The MRDPND signal istrue during state E, while the MRDBPND signal is true during state F.

The MCM module 412 is also responsible for indicating when data is to belatched into the data buffer 158 from the processor data bus on writecycles. In the case of single write operations data is latched intolatches 272 and 274, while in burst operations all four latches areutilized. However, it is noted that in a single write case the DPMLE1signal is actually not generated as the decode later indicates that onlya single write is necessary.

Upon reset of the system the MPMLE or processor to memory latch enablestate machine shown in FIG. 19 starts at state A. The signals referredto as SET₋₋ DPMLE0 and SET₋₋ DPMLE1 are active in state A when theMWEPBRDY signal or memory write early PBRDY signal is provided.Additionally, the DPMLE1 signal is provided in state D when the MWEPBRDYsignal is active. Control proceeds from state A to state C when theMWEPBRDY signal is received from the MWEPBRDY state machine (FIG. 21)indicating that this write cycle has been posted and it is appropriateto proceed to the next cycle. When the MWEPBRDY signal has not beenreceived, control remains at state A so that the first two latchesremain open. Control proceeds from state C to state D if the MPROG statemachine has indicated that this is a burst write situation and the firstportion is proceeding as indicated by the MWRB1 signal. Otherwisecontrol proceeds from state C back to state A. If the burst operation isoccurring, the CLR DPMLE1 signal is asserted in state C so that thelatch signal is inactivated so that it can be made active again in stateD. Control remains in state D until the MWEPBRDY signal is provided,indicating that the next of the write cycles has been posted. Otherwisecontrol remains in state D. Additionally in state D the SET₋₋ PMDRDYsignal or set processor memory ready signal is provided when theMWEPBRDY signal is indicated so that a PMRDY signal can be provided tocommence the actual write cycle to the memory. Control proceeds fromstate E to state F in all cases. State F is utilized for the third writecycle in the burst sequence and control remains at state F while theMWEPBRDY signal is not being asserted, that is, to indicate that thiscycle has not been successfully posted. When state F is active and theMWEPBRDY signal is asserted the SET₋₋ DPMLE2 signal is asserted so thatthe latch 276 will be closed. Control proceeds from state F to state Gwhen the posting of the cycle has been indicated. Control proceeds fromstate G to state H after a single PCLK signal cycle. State H is thefinal of the four write cycles in the write-back operation and controlremains in state H until the MWEPBRDY signal is asserted, at which timecontrol proceeds to state A. The SET₋₋ DPMLE3 signal is asserted instate H when the MWEPBRDY signal is provided. Additionally, the SET₋₋PMDRDY signal is provided in state H when the MWEPBRDY signal isprovided to indicate full width memory data can be provided to thememory.

As indicated in the discussion of the MPMLE state machine, the signalSET₋₋ PMDRDY indicates that data has been transferred into the postingbuffers and can be transferred. This signal is utilized by the PMDRDYstate machine shown in FIG. 20. This state machine commences operationin state A after reset. If the SET₋₋ PMDRDY signal is received, controlproceeds from state A to state B. Otherwise control remains in state A.From state B, control proceeds to state C if the SET₋₋ PMDRDY signal isactive and there is no indication to clear PMRDY signal as beingprovided by the CLR₋₋ PMRDY, signal. The CLR₋₋ PMRDY signal is theANDing of the MRDY signal and the OR of the MWR, MWRB1 and MWRB21signals. Thus, clearing the PMRDY signal is appropriate if the memoryhas indicated that it is ready and a write operation, either single orburst, is indicated as being in progress. Control proceeds from state Bto state A if clearing of the PMDRDY signal is appropriate and the SET₋₋PMDRDY signal is not active. Otherwise control remains in state B.Control proceeds from state C to state B if it is appropriate to clearthe PMDRDY signal and its not set. Otherwise, control remains in stateC. The PMDRDY state machine thus provides PMDRDY signal to indicate tothe memory control section that the write posting buffer has beenfilled. The PMDRDY signal is active in states B and C or when the MWR₋₋PMDRDY signal is provided by the MPMLE state machine, which is true whenthat state machine is in state A, the MWEPBRDY signal has been receivedand this is a single write operation. Thus the operation commences instate A and proceeds to state B when the first 128 bits is available andto state C if a second 128 bits is available and the first 128 bits hasnot been actually written to the memory system. As each 128 bits isactually written the PMDRDY state machine backtracks from state C tostate B to state A.

As noted above, the transitions in the MPMLE state machine are basedprimarily on the presence of the MWEPBRDY signal. This signal isprovided by the MWEPBRDY state machine shown in FIG. 21. Upon reset thisstate machine starts in state A, the conventional idle state. Controlproceeds from state A to state C if the SPADS signal is active, a memorydata write cycle is decoded and either a processor cycle is not inprogress or a processor write cycle is completing or, in a secondalternative, if the DSPADS signal is present, a main memory data writecycle is decoded and a main memory data write cycle is in progress andcompleting. Control proceeds from state A to state F if the DSPADSsignal is active, a local memory data write cycle is decoded, a readcycle is in progress and has not completed. Control proceeds from stateA to state G if the DSPADS signal is active, a main memory data writecycle has been decoded and a memory read cycle is completing, or if theSPADS signal is active, a memory data write cycle has been decoded and aread cycle is completing. Control proceeds from state F to state G whenthe PPEOC signal is received and otherwise remains in state F. Controlalways proceeds from state G to state C. Control proceeds from state Ato state H if the DSPADS signal is active, a main memory data writecycle has been decoded and a memory write cycle is active but notcompleting. In all other cases control remains at state A. Controlproceeds from state H to state C when the PPEOC or end of cycle signalis received and otherwise remains in state H.

Control proceeds from state C to state D if a main memory data writecycle is decoded and otherwise returns to state A. Control proceeds fromstate D to state E if the first portion of a write-back is in progressas indicated by the MWRB1 signal or if a write-back is pending. Controlproceeds from state D to state A if a memory cycle is not in progressand a memory write cycle is not pending or if a processor cycle is notin progress. In all other cases control proceeds from state D to stateJ. Control proceeds from state E to state J if the PEOC signal isactive, indicating that the cycle is completing. In all other casescontrol proceeds from state E to state I. Control always proceeds fromstate I to state E.

There are numerous exits from state J. Control proceeds from state J tostate C if the SPADS signal is active, a memory data write is decodedand a memory cycle is not about to commence, a cache cycle is notoccurring, and a host bus cycle is not occurring and if a processor readcycle is not occurring or, as a second condition, if a single writecycle or a first portion of a write-back cycle is occurring, a cachecycle is not in progress and the currently in progress write cycle isabout to complete or, as a third condition, if a memory write cycle ormemory burst write cycle is pending, a write cycle is in progress and noother cycles are in progress. Control proceeds from state J to state Fif the DSPADS signal is active, a main memory data write cycle isdecoded, a memory cycle is not in progress, the cycle is not completedto the processor and either a cache cycle is in progress or a host buscycle is in progress. Control proceeds from state J to state G if theDSPADS signal is true, a main memory write data cycle has been decoded,a memory cycle is not in progress and the current cycle is completing.Control proceeds from state J to state A if a memory cycle is not inprogress, a memory write cycle is not pending, a memory burst writecycle is not pending and both the SPADS and DSPADS signals are notasserted. In all other cases, control remains at state J.

The MWEPRDY signal is asserted by this state machine when the statemachine is in state C, a main memory data write cycle is decoded and thewrite cycle is not to a write protected memory location as indicated bythe HWP signal being low or the SMIACT signal being high. Further, theMWEPBRDY signal is also active during state E. The MWEPBRDY signal isconsidered an early ready signal for local memory write operations andit will be combined with other early ready signals and resynchronized toactually provide the PBRDY signal. As a result, it is provided one PCLKsignal cycle earlier then the PBRDY signal and so is considered early.The assembly of these early ready signals is described below.

The memory control module 412 is also responsible for developing anearly ready signal for main memory read operations. This signal isprovided by the MREPBRDY state machine as shown in FIG. 22. This statemachine begins operation on reset at state A. Control proceeds fromstate A to state B if a host bus cycle has not already started, a cachecycle is not in progress and either the memory is ready and a singlememory read cycle has occurred or the memory is ready and a memory readburst cycle is occurring or, as a second alternative, if a memory earlyready start signal has been received. In all other cases control remainsat state A. The development of the MRERDY₋₋ STRT signal is shown in FIG.38I.

The MRDHLD and MRDBHLD signals are provided to an OR gate 820. Theoutput of the OR gate 820 is combined with the SHPROG₋₋ 1ST and HPROG*signals by AND gate 822. The MRDHLD and MRDBHLD signals are alsoprovided to an OR gate 824. The output of the OR gate 824 and the CPROG*and HPROG₋₋ 1ST* signals are combined by AND gate 826. An OR gate 828receives the outputs of the AND gates 822 and 826 and provides an outputto the D input of a D-type flip-flop 830, which is clocked by the PCLKsignal. The SHPROG₋₋ 1ST, MEMRD and SMRDY signals are combined by an ORgate 832, whose output is combined with the MRDHLD and MRDBHLD signalsby OR gate 834. The output of the OR gate 834 is combined with theCPROG* and HPROG₋₋ 1ST* signals in an AND gate 836. The noninvertedoutput of the flip-flop 830 and the output of the AND gate 836 are theinputs to an OR gate 838, whose output is the MRERDY₋₋ STRT signal.

Control proceeds from state B to state C if this is a memory read burstcycle or a memory read burst cycle is being held for host or cache busoperations. In all other cases control proceeds from state B to state A.Control proceeds from state C to state D in all cases and from state Dto state E in all cases. Control remains at state E until the MRDY ormemory ready signal is received from the memory controller block 402 andat that time proceeds to state F. Control then proceeds sequentiallyfrom state F to state G to state H to state A on successive PCLK signalcycles.

The MREPBRDY or memory read early PBRDY signal is asserted in state B,D, F and H as appropriate for single and burst operations. The MRDBHLDexiting signal, referred to as MRDBHLD₋₋ LV, is provided in state C tothe MPROG state machine as previously described.

In addition, certain miscellaneous logic is present in the memorycontrol module 412. One of the functions of this miscellaneous logic isto produce the PDMDLE*<3 . . 0> signals from the SET₋₋ DPMLE<3 . . 0>signals. The development of these signals is shown in FIG. 23. The MWRsignal, asserted in state H of the MPROG state machine, is an input to atwo input OR gate 500 and a two input OR gate 502. The MWRB1 signal orfirst portion of a write-back cycle is provided to the second input ofthe OR gate 500. The MWRB2 or second write-back cycle portion signal,asserted in state J of the MPROG state machine, is provided to thesecond input of the OR gate 502 and indicates the second 128 bits of awrite-back cycle. The MRDY or memory ready signal from the memorycontroller block 402 is provided as one input to two NAND gates 504 and506. The second input to the NAND gate 504 is the output of the OR gate500, while the second input to the NAND gate 506 is the output of the ORgate 502. The output of the NAND gate 504 is one input to an AND gate508, whose second input is the noninverted output of a D-type flip-flop510. The output of the AND gate 508 is provided to one input of a twoinput OR gate 512, whose second input receives the SET₋₋ DPMLE0 signal.The output of the OR gate 512 is provided to the D input of theflip-flop 510. The noninverted output of the flip-flop 510 is thePDMDLE*<0> signal.

The output of the NAND gate 504 is also provided to an inverted input ofa two input NOR gate 514, whose second input is not inverted andreceives the CLR₋₋ DPMLE1 signal. The output of the NOR gate 514 isprovided as one input to a two input AND gate 516, whose second inputreceives the noninverted output of a D-type flip-flop 518. The output ofthe AND gate 516 is provided as one input to a two input OR gate 520,whose second input receives the SET₋₋ DPMLE1 signal. The output of theOR gate 520 is provided to the D input of the flip-flop 518. Thenoninverted output of the flip-flop 518 is the PDMDLE*<1> signal.

The output of the NAND gate 506 is provided as one input to a two inputAND gate 522, with the second input to the AND gate 522 provided by thenoninverted output of a D-type flip-flop 524. The output of the AND gate522 is provided as one input to an OR gate 526, whose second input isthe SET₋₋ DPMLE2 signal. The output of the OR gate 524 is provided tothe D input of the flip-flop 524. The noninverted output of theflip-flop 524 is the PDMDLE*<2> signal. The output of the NAND gate 506is also provided to one input of a 2 input AND gate 528, whose secondinput receives the inverted output of a D-type flip-flop 530. The outputof the AND gate 528 is provided to one input of a two input OR gate 532,whose second input receives the SET₋₋ DPMLE3 signal. The output of theOR gate 532 is provided to the D input of the flip-flop 530. Thenoninverted output of the flip-flop 530 is the PDMDLE*<3> signal. ThePCLK signal is provided as the clocking input to the flip-flops 510,518, 524 and 530.

In a manner similar to the memory control module 412, the host controlmodule 414 also includes a series of state machines. The first of theseis the HRD or host read state machine shown in FIG. 24. The statemachine tracks the read cycles, either single or burst, that are runningor executing on the host bus H. The HRD state machine starts at state Aupon reset. Control proceeds from state A to state B when the DSPADSsignal is active, a host cycle read which is not a burst is requestedand either a host write cycle is in progress or one is pending. Controlproceeds from state A to state F if the DSPADS signal is active, a hostread cycle is requested, it is a burst cycle and either a host writecycle is in progress or one is pending. Control proceeds from state A tostate C for requested host read, non-burst cycles when the DSPADS signalis active and neither a host write cycle is in progress nor is onepending. Control proceeds from state A to state D on requests for hostread, burst cycles when the DSPADS signal is asserted and a host writecycle is not in progress and one is not pending. In all other casescontrol remains at state A.

Control proceeds from state B to state C if the host ready signal hasbeen provided or a host write is not in progress and a host write is notpending. In all other cases control remains at state B. Control proceedsfrom state F to state D if either a ready signal is received from thehost controller block 404 or a write cycle is not in progress, and ahost write cycle is not pending. In all other cases control remains atstate F.

Control proceeds from state C to state E if the HREPBRDY or host readearly PBRDY signal is asserted and either a host read cycle is pending,a host read burst cycle is pending, or the DSPADS signal is asserted anda host read cycle is being asserted. Control proceeds from state C tostate A if the HREPBRDY signal is asserted and it is none of the furtherconditions mentioned for transfer to state E. In all other cases controlremains at state C. Control proceeds from state E to state C if a hostread is pending and otherwise proceeds to state D. Control proceeds fromstate D to state E if the HREPBRDY signal is present, the PEOC signal isactive and either a host read is pending, a host burst read is pendingor a host read cycle has been sampled on the processor bus. Controlproceeds from state D to state A if the HREPBRDY signal is present, thePEOC signal is present and none of the additional conditions in thetransfer from state D to state E are present. In all other cases controlremains at state D. Host cycles are identified as either I/O cycles ormemory cycles where the HLOCMEM* signal is high, indicating a non-mainmemory location.

Two signals are provided by the HRD state machine. These are the HRDPRGsignal or host read in progress signal, which is active during states B,C and D, and the HOSTRD or host read signal, which is active duringstates B, C or D and when a host cycle is active and the processor isindicating a read request.

As noted, the HRD state machine utilizes as two of its conditions theHRDPND and HRDPBND or host read pending and host read burst pendingsignals. These are provided by the HRDPND state machine of FIG. 25. TheHRDPND state machine commences at state A at reset. Control proceedsfrom state A to state B when the DSPADS signal is asserted, a host readcycle is in progress and a host non-burst read cycle is being requestedby the processor 152. Control proceeds from state A to state C if theDSPADS signal is asserted, a host read cycle is in progress and a hostburst read cycle is being requested by the processor 152 to the host busH. In all other cases control remains at state A. Control proceeds fromstate B to state A when the HRDPRG or host read in progress signal isdeasserted. In all other cases control remains at state B. Controlproceeds from state C to state A when the HRDPRG signal is deasserted.In all other cases control remains at state C. The HRDPND or host readpending signal is asserted in state B, while the HRDBPND signal isasserted in state C.

Just as the HRD state machine is used to track host read cycles,similarly a state machine is used to track host write cycles. This isthe HWT state machine as shown in FIG. 26. The machine enters state A atreset and proceeds from state A to state C if a host write cycle isdecoded, the previous cycle has been acknowledged as indicated by theHACK signal being low, write posting is enabled and a processor cycle isnot pending. Control proceeds from state A to state G if a host writecycle is indicated, posting is not enabled and a processor cycle is notpending, or if a write to the host bus is requested, a processor cycleis pending and ending, and posting is not enabled. Control proceeds fromstate A to state F if a host write cycle is requested, a processor cycleis pending and ending, and posting is enabled. Control proceeds fromstate A to state E if a host write cycle is requested and a processorcycle is pending and has not ended. In all other cases control remainsat state A.

Control proceeds from state G to state C if a host write cycle pendingor if a host write cycle is requested and posting is enabled, and ineither case if an SHRDY or synchronized HRDY signal is indicated, sothat the host bus is ready to receive the write operation. The HRDYsignal is provided at the noninverted output of a D-type flip-flop,which is clocked by the PCLK signal. The output of a multiplexor isconnected to the D input, with the PHASE signal providing the selection.The zero input receives the noninverted output of the flip-flop, whilethe one input receives the HERDY signal, which is the inverted versionof the HERDY* signal present on the host bus H to indicate a singlecycle early ready indication. Control proceeds from state E to state Gif a processor cycle is ending, as indicated by the PEOC and PBRDY ofSEOC signals and posting has been disabled. Control proceeds from stateE to state F when the processor cycle is completing and posting isenabled. In all other cases control remains at state E. Control proceedsfrom state F to state C in all cases. Control proceeds from state C tostate D in all cases.

There are several exits from state D. The first exit is from state D tostate E, which occurs when a ready is received, the host write cycle isbeing requested or is pending, either a processor write cycle is not inprogress or the host write cycle was not first, and if the processorcycle which is pending is not completed. Control proceeds from state Dto state F upon receipt of the SHRDY signal, either a write is pendingor a write is requested and posting is enabled, and a read cycle whichis in progress by the processor is ending. Control proceeds from state Dto state C when ready is received and either another write cycle ispending or is being requested and posting is enabled. Control proceedsfrom state D to state G if the ready is received and a non-posted writecycle is pending or a write cycle is pending and posting is disabled.Control proceeds from state D to state A if the ready is received, nowrite cycle is pending and no write cycle is being requested. The HWTstate machine also indicates when host write cycles are in progress andwhen the ready signal is to be provided. The HWTPRG or host write inprogress signal is present, when the HWT state machine is in other thanstate A. The HWEPBRDY or host write early processor ready signal isprovided in state C or in state G when the HRDY or host ready signal hasbeen returned. The SET₋₋ DPHLE signal is active during state C.

As noted, the HWT state machine needs to know if a write cycle ispending. This is done by the HWL1PND state machine of FIG. 27. The statemachine commences at state A upon reset. Control proceeds from state Ato state B if a host write cycle is in progress, a host write cycle isbeing requested, posting is enabled and the previous write cycle has notcompleted or if a second level of write cycle is posted and pending.Control proceeds from state A to state C when the DSPADS signal is true,a write cycle is in progress, a write cycle is being requested, thewrite cycle in progress has not completed and posting is disabled or ifa second non-posted write cycle is pending. In all other cases controlremains at state A. Control proceeds from state B to state A when aready is received from the host controller 404. In all other casescontrol remains at state B. Control proceeds from state C to state Awhen a ready is returned and otherwise remains at state C. The HWTL1PNDor first level host write pending signal is active during state B, whilethe HWTNPL1 ND or host first level not posted write cycle pending istrue in state C.

As just mentioned, it is possible to have three host bus write cyclespending at one time. This is possible because one cycle can be posted tothe host bus H, one cycle can be posted to the EISA bus E from the hostbus H and another one can be pending from the processor 152. Thereforeit is necessary to track to two levels. This tracking is done by theHWL2 PND state machine of FIG. 28. This state machine commences at stateA upon reset and proceeds to state B when the DSPADS signal is active, awrite cycle to the host bus is being requested and posting is enabledand either a first write cycle, either posted or not posted, is pending.Control proceeds from state A to state C if the DSPADS signal is active,a host write cycle is being requested, posting of that cycle is notenabled and a cycle is already pending. In all other cases, controlremains at state A. Control proceeds from state B to state A when aposted or non-posted write cycle is not pending. That is, the previouslypending cycle is no longer pending but has completed. In other casescontrol remains at state B. Similarly, control proceeds from state C tostate A if the prior pending signal has been performed. Control remainsat state C otherwise. The HWTL2 PND or host write posted level 2 pendingsignal is active in state B, while the HWTNPL2 PND or level 2 writepending non-posted signal is active in state C.

Just as a processor ready signal is provided in write cases, it is alsonecessary to provide an early processor ready in read cases. This isdone by the HREPBRDY state machine of FIG. 29. This state machinecommences at state A upon reset. Control proceeds to state B if a hostread is in progress, a host write is not in progress, a processor cycleis pending, the current processor cycle is not ending and the host buscycle which is in progress was not commenced prior to the processorcycle which is in progress. In all other cases control remains at stateA. Control proceeds from state B to state C if the current processorcycle is completing and the host bus cycle has not completed. Controlproceeds from state B to state D if the host bus cycle is completing andthe processor cycle is not completing. Control proceeds from state B tostate E if both the host and processor cycles are completing. Controlotherwise remains at state B. Control proceeds from state C to state Awhen the host cycle is completing. Otherwise control remains at state C.Control proceeds from state D to state E when the processor cycle iscompleting. Otherwise control remains at state D. Control alwaysproceeds from state E to state A.

The HREPBRDY or host read early PBRDY signal is provided in state E, instate C if the host bus cycle is being completed or in state A if a hostwrite cycle is not in progress, the host cycle that was in progress wasfirst and it is being completed.

Certain miscellaneous logic also contained in the host control module414 is shown in FIG. 51. The PDHDLE* signal is provided as the invertedoutput of a D-type flip-flop 700. The PDHDLE* signal is also connectedto an OR gate 702. The OR gate 702 receives the SET DPHLE signal fromthe HWT state machine as its other input. The output of the OR gate 702is ANDed with an indication of the inverted HRDY signal in NAND gate704. The output of the NAND gate 704 is provided to the D input of theflip-flop 700. The flip-flop 700 is clocked by the PCLK signal. Thelogic also develops the HPROG or host bus cycle in progress signal. Thisis developed as the output of an OR gate 706. The OR gate 706 receivesthe output of an OR gate 708 and the noninverted output of a D-typeflip-flop 710. The OR gate 708 receives the HRDPRG host read in progresssignal, the HWTPRG host write in progress signal and the HWL1PEND hostwrite pending signals. The output of the OR gate 706 is one input to anAND gate 712, with the second input receiving the output of a NOR gate714 which in turn receives the PEOC and PBRDY signals to indicate thatthe current processor cycle is not completing. The output of the ANDgate 712 is provided as the D input of the flip-flop 710. The flip-flop710 is clocked by the PCLK signal. The final signal of interest producedby the HCM module 414 is the ignore parity check or IGNPCHK signal. Thissignal is utilized by the processor 152 during read operations from thehost bus H because parity is not maintained on the host bus H. TheIGNPCHK signal is provided as the noninverted output of a flip-flop 716whose input receives the output of a two input OR gate 718. One input tothe OR gate 718 is provided by the noninverted output of a D-typeflip-flop 720. The second input is provided by the noninverted output ofa D-type flip-flop 722, which is also connected to the D input of theflip-flop 720. The HREPBRDY signal is provided to the D input of theflip-flop 722 and all three flip-flops 716, 720 and 722 are clocked bythe PCLK signal.

Upon reviewing the data buffer 158 it is seen that the FECON module 400must also provide the PDSEL<1 . . 0> signals to allow proper control ofthe multiplexor 338 during read cases. This is done primarily by the useof the DPSEL state machine shown in FIG. 30. This state machinecommences at reset in state A and proceeds to state C if a host readcycle is in progress (as indicated by the HRD state machine being instates B, C or D or if a host read cycle is occurring) and will bereturning its data first because the host cycle was started first asindicated by the HPROG₋₋ 1st signal. Control proceeds from state A tostate B under several different conditions. The first of those is if amemory read cycle is in progress, the early PBRDY signal has beenprovided, the cycle has not completed and there is not a host read cyclein progress which was in progress first. A second condition for atransfer from state A to state B is if a memory read cycle is occurring,the early PBRDY is provided, the cycle is ending and the cycle which isbeing completed had the PA<3> bit cleared, as indicated by the LPAI03signal. The final condition of transfer from state A to state B is if amemory read cycle is occurring as indicated by the MEMRD and MCYCsignals, the memory cycle has been acknowledged by the memory controllerblock 402, the PA<3> signal was cleared to indicate the lower 64 bitsand the PBRDY signal has not been asserted. In all other cases controlremains at state A.

Control proceeds from state B to state C if a host read cycle which wasstarted first is occurring. Control proceeds from state B to state A forthe same conditions which were true for transfers from state A to stateB except that the PA<3> bit would be set as utilized in those particularconditions. Otherwise control remains at state B. Control proceeds fromstate C to state A when a host read cycle is not occurring, a memoryread cycle is occurring and bit PA<3> is not set. Control proceeds fromstate C to state B if a host read cycle is not occurring, a memory readcycle is occurring and the PA<3> bit is set. Otherwise control remainsat state C.

The particular state of the DPSEL state machine determines the selectionof the multiplexor. In state A the signals referred to as NPDSEL<1 . .0> are set to indicate the first channel. In state B the NDPSEL<1 . . 0>signals are set to indicate the second channel, while in state C theNDPSEL<1 . . 0> signals are set to indicate the third input of themultiplexor 338. The NDPSEL<1 . . 0> signals are provided to the inputsof two D-type flip-flops, with the PDSEL<1 . . 0> signals produced bythe non-inverting outputs of the flip-flop, with the flip-flops beingclocked by the PCLK signal.

One signal which must be provided to the processor 152 to allowpipelining is a signal referred to as PNA or processor next address. Theprocessor 152 sees this signal and knows that it can provide the nextaddress in a pipelined mode. Therefore it is necessary to properlygenerate the PNA signal. A series of state machines and random logic areutilized to provide this signal. The first state machine is the PNAstate machine of FIG. 31. This state machine is used to provide anindication that a PNA signal can be provided within a given window. Thestate machine begins at state A upon reset. Control proceeds from stateA to state B if the SPADS signal is true and a signal referred to asEPNA DEC or early PNA decode is provided. The EPNA DEC signal isdeveloped under two conditions. The first is a processor read cyclewhich is not to a local, i.e. processor bus, location and is not aspecial cycle. The second is that a non-special, non-local processorwrite cycle which is not being responded to by the L2 cache 154 and isnot a code write is being requested. Otherwise control remains at stateA. Control proceeds from state B back to state A if the PPEOC signal isasserted, indicating that the cycle is completing. Control proceeds fromstate B to state C if the PNA signal is asserted and the cycle is notcompleted. Control remains at state B in all other cases. Controlproceeds from state C back to state A if the SPADS signal is assertedand the EPNA₋₋ DEC signal is not asserted or if the cycle is ending.Control proceeds from state C back to state B if the SPADS signal isasserted, the current cycle is ending and the EPNA₋₋ DECWR signal orwrite condition noted above is not true or a read is not in progress.Control proceeds from state C to state E if the SPADS signal is active,the current cycle is completing and the EPNA₋₋ DEC signal is active.Control proceeds from state C to state D if the SPADS signal is active,the cycle currently in progress is not completing and the EPNA₋₋ DECsignal is asserted. In all other cases control remains at state C. Beingin state D is an indication that the operation is pipelined. Controlproceeds from state D to state B if the PPEOC signal is asserted,indicating that a cycle is completing. Otherwise control remains atstate D. Control proceeds from state E to state B in all cases.

The PNA state machine provides one signal and this is the PNA₋₋ OKsignal when the state machine is in state B. This is an indication thatit is possible to receive another address as one cycle is not currentlypipelined or is effectively completing.

A state machine referred to as the RDPNA state machine is shown in FIG.32 and is used to indicate that the PNA signal can be generated for thecurrent read cycle. Control commences at state A upon reset. Controlproceeds from state A to state B if the signal EPNA₋₋ DECRD, the readterm of the EPNA₋₋ DEC signal, is true and any of the following sets ofconditions is true. First, if a MCYC or memory cycle is indicated and anacknowledgement has been received from the memory control block 402.Second, if a host cycle is active and an acknowledgement has beenreceived from the host control block 404. Third, if the DSPADS signal isactive and the cycle is neither a memory cycle or a host cycle. In allother cases control remains at state A. Control proceeds from state Bback to state A if the PNA signal is asserted or if the current cycle iscompleting as indicated by the PPEOC signal. Otherwise control remainsin state B. The RDPNA₋₋ OK signal is generated in state B or when thenext state is going to be state B.

A similar state machine, the WRPNA state machine, is shown in FIG. 33and is used to generate the WRPNA₋₋ OK signal for write cycle cases.This state machine begins at state A on reset and proceeds to state Bunder one of three conditions. The first is that the SPADS signal isasserted, the EPNA₋₋ DECWR signal is asserted, a memory write is notoccurring and either a host write is not in progress or the host controlblock 402 has acknowledged an operation. The second condition is if theSPADS signal and the EPNA₋₋ DEC signals are both asserted, a memorywrite cycle is in progress and has been acknowledged as completing soonby the EMACK signal and either a host bus write is not in progress or ithas been acknowledged. The final condition is if a host cycle isoccurring, it is has been acknowledged, it is a host write which is inprogress, a PNA signal has not been sent and the EPNA₋₋ DECWR signal hasbeen asserted. In all other cases control remains at state A. Controlreturns from state B to state A if the PNA signal has been asserted orif the cycle has ended as indicated by the PPEOC signal. Otherwisecontrol remains at state B. The WRPNA₋₋ OK signal is developed if thestate machine is in state B or if the next state is state B.

The PNAGEN module 416 includes some random logic to generate the actualPNA signal. The PNA signal is generated by the non-inverting output of aflip-flop, with the PNA* signal generated by the inverted output of theflip-flop. The flip-flop is clocked by the PCLK signal. The input to theflip-flop is provided by the output of an OR gate which has inputs ofthe PNA₋₋ OK signal, the HPNA₋₋ OK signal and the output of an OR gatewhich combines the RDPNA₋₋ OK and WRPNA₋₋ OK signals. The HPNA₋₋ OKsignal is the output of a NAND gate which has as its inputs a signalindicating that host write cycle is in progress and the output of an ORgate, which has as one of its inputs a signal indicating that a hostread cycle is in progress and as its second input an AND gate which hasas its inputs a signal which indicates that a host cycle is present andthe EPNA₋₋ DECRD signal.

As indicated above, numerous of the state machines use the PBRDY orprocessor burst ready signal to advance. Certain special cycles such asbranch trace messages as provided by the processor 152 must not beprovided to the host bus H to prevent errors, but ready indication mustbe provided. Additionally, the processor 152 must receive readyindication for write operations to write-protected areas, even though noactual operation will occur. The EPBRDY signal for these particularcases is provided by the WPEPBRDY state machine shown in FIG. 34. Thestate machine begins at state A upon reset and transfers to state B ifthe DSPADS signal is true, a data write to local or main memory isindicated, SMI mode is not active and write protect status is indicatedfrom the DDF 164 or if the DSPADS signal is true and a branch tracemessage has been decoded, with the additional condition in both casesthat a processor cycle is not in progress in the memory controller 156or that a cycle which is in progress is ending and the PBRDY signal isasserted. Control proceeds from state A to state C if the DSPADS signalis asserted, a local memory write data operation is indicated, SMI modeis not active and the write protect indication is received from the DDF164 or if the DSPADS signal is true and a branch trace message has beendecoded, both of these combined with the fact that a processor cycle isin progress. Otherwise control remains at state A. Control alwaysproceeds from state B to state A. Control proceeds from state C to stateB when the cycle in progress is completed as indicated by the PEOC andPBRDY signals being asserted. Otherwise control remains at state C. TheWPEPBRDY signal is asserted in state B. Thus this state machine is usedto provide the processor BRDY indications in the case of writes to writeprotected memory or on branch trace messages. In the case of the writesto write protected areas none of the other state machines perform thisoperation and the data is actually ignored. Similarly, the data isignored for branch trace messages and they are not passed to the hostbus H.

With the development of this final EPBRDY signal, it is appropriate todescribe the logic which develops the PBRDY signal. The signal referredto as EPBRDY signal is developed by ORing together the MREPBRDY,MWEPRDY, HREPBRDY, HWEPBRDY and WPEPBRDY signals. This signal is ANDedwith an indication that the cache controller is not performing a cacheline fill and provided to the input of a D-type flip-flop. Thenoninverted output of the flip-flop is the PBRDY signal, while theinverted output of the flip-flop is the PBRDY* signal which is providedto the processor 152.

As previously noted, two signals referred to as MCYC and HCYC have beenutilized. As indicated in FIG. 10 the MCYC signal is provided to MCONblock 402 and HCYC signal is provided the HCON block 404 to indicatethat they have cycles to perform. This decoding is done by certainlogic.

The MCYC logic is shown in FIG. 35. A six input AND gate 550 receivesthe PMIO, PDC, PWR*, PLOCAL*, CHIT₋₋ OK* and HLOCMEM signals to decode amain memory data read cycle which is not an L2 cache 154 hit. A sixinput AND gate 552 receives the PMIO, PDC*, PWR*, PLOCAL*, HLOCMEM andCHIT₋₋ OK* signals to decode a memory command read cycle which is not aL2 cache 154 hit. A six input AND gate 554 receives the PMIO, PDC, PWR,PLOCAL* and HLOCMEM signals and the output of a two input OR gate 556which receives the SMIACT and HWP* signals. Thus this AND gate 554decodes a memory data write cycle. It is noted that the SMIACT and HWP*signals are ORed in the OR gate 556. This is done so that areas ofmemory which are normally write protected, such as the system managementmemory, are not write protected during system management mode. Thus anarea of conventional memory can be utilized as the SMM area withoutdanger of being over written and yet it is fully accessible during SMMmode.

The outputs of the AND gates 550, 552, 554 are provided to the inputs ofa three input OR gate 557 whose output is provided as one input to a twoinput AND gate 558. The second input of the AND gate 558 receives theDSPADS signal. The output of the AND gate 558 is provided as one inputto a two input OR 560, whose second input receives the noninvertedoutput of a D-type flip-flop 562. The output of the OR gate 560 is theMCYC signal and is provided to the input of a two input AND gate 564.The second input of the AND gate 564 is inverted and receives the MACKsignal. The output of the AND gate 564 is provided to the D input of theflip-flop 562, which flip-flop 562 is clocked by the PCLK signal.

The development of the HCYC signal is shown in FIG. 36. A four input ANDgate 570 receives the CHIT₋₋ OK*, PMIO, PDC, and PWR signals to decode ahost bus memory data write operation. A five input AND gate 572 receivesthe PMIO, PDC*, PWR*, HLOCMEM* and HLOCMEM* signals to decode a host busmemory command read operation which is not a cache hit. A five input ANDgate 574 receives the PMIO, PDC, PWR*, HLOCMEM* and CHIT₋₋ OK* signalsto decode a host bus memory data read operation which is not a cachehit. A 2 input AND gate 576 receives the PMIO* and PWR* signals todecode host bus I/O read operations. A three input AND gate 578 receivesthe PMIO*, PDC and PWR signals to indicate a host bus I/O data writeoperation. The outputs of the AND gates 570, 572 and 574 and the ANDgates 576 and 578 are provided as inputs to a seven input OR gate 580.The PLOCAL and HSPCYC signals, which indicate respectively processor buslocal cycles and special cycles which are to be transmitted to the hostbus, that is 80486 equivalent special cycles, provide the remaininginputs to the OR gate 580. The output of the OR gate 580 is provided asone input to a two input AND gate 582, whose second input receives theDSPADS signal. The output of the AND gate 582 is provided as one inputto a two input OR gate 584. The second input to the OR gate is connectedto the non-inverting output of a D-type flip-flop 586. The flip-flop 586is clocked by the PCLK signal. The output of the OR gate 584 is providedas one input to a two input AND gate 588, whose second input is invertedand receives the output of a two input AND gate 590. The output of theAND gate 588 is provided to the D input of the flip-flop 586. The HACKsignal is provided as one input to the AND gate 590 and the output of atwo input NAND gate 592 provides the second input. The PPROGRD and PWRsignals are the inputs to the NAND gate 592. The output of the NANDgate. 592 is also provided as one input to a 2 input AND gate 594, whosesecond input receives the output of the OR gate 584. The output of theAND gate 594 is the HCYC signal. This completes the description of thefront end controller 400.

The memory control block 402 is shown in more detail in FIG. 12 and itsstate machines will be discussed at this time. The CPUMEM module 430 isresponsible for the actual control of memory operations directed fromthe processor 152. The core of the CPUMEM module 430 is the MEM ormemory state machine as shown in FIG. 37. Control proceeds to state MIor memory idle upon reset. In this state the state machine is idle andis precharging the DRAMs and the MRAS* signal is high. Control proceedsfrom state MI to state MRC if the MCYC signal is active, indicating thata memory cycle is being requested, and the CHGTO* or not charge time outsignal is asserted, indicating that the DRAMs have not been fullyprecharged. Control proceeds from state MI to state MR1 or memory RAS1if a cycle has been requested, as indicated by the MCYC signal, and theCHGTO signal is true, indicating that the precharge time has completed.Control proceeds from state MI to state MN when the BMPROG or bus mastercycle in progress signal is asserted. In all other cases control remainsat state MI. Control proceeds from state MRC to state MR1 when the CHGTOsignal is active, indicating that the precharge time has been completed.Control proceeds from state MR1 to state MR2 in all cases. Controlproceeds from state MR2 to state MC1, the first of the CAS states, ifthe DRAMs have a speed of 60 ns, as indicated by the LSPD60 or latchedSPD60 signal from the DDF 164, and either a read cycle has beenindicated or the PMDRDY or processor to memory data ready signal isactive. Control proceeds from state MR2 to state MR4 for 60 ns DRAMs ifa write cycle has been requested and the PMDRDY signal is not asserted.In all other cases control proceeds from state MR2 to MR3. Particularly,control proceeds from state MR2 to MR3 in the case of 80 ns DRAMs.Control proceeds from state MR3 to state MR4 in all cases. Controlproceeds from MR4 to state MC1 if either a read cycle has been requestedor the data is available as indicated by the PMDRDY signal. Controlremains at state MR4 for all other cases.

Control always proceeds from state MC1 to state MC2. Control proceedsfrom state MC2 to state MC3 when the MPDREQ signal is asserted,indicating that the FECON block 400 is ready to accept data from thememory. In all other cases control remains at state MC2. Control alwaysproceeds from state MC3 to state MN.

There are numerous exits from state MN. In state MN the MRAS* signal islow. Control proceeds from state MN to state MC1 if a burst cycle isoccurring, if a non-burst memory page hit read has occurred as indicatedby the MCYC, BURST*, MWR* and PGHIT signals being true or if the nextcycle is a non-burst memory page hit, the previous cycle was a memorywrite, and the data is available, as indicated by the MCYC signal,BURST*, PGHIT, PMDRDY and PRMWR signals being asserted. The PRMWR signalindicates the previous memory cycle is a write cycle. This loop fromstate MN to state MC1 is the shortest loop and indicates a two waitstate loop based on a 66 MHz clock. Thus, once page hits are obtained,the memory controller 156 can operate in two wait state per memory cyclemode for most page hit operations. Control proceeds from state MN tostate MR4 on non-burst memory page hit writes where the data is notavailable, as indicated by the MCYC, BURST*, MWR, PGHIT and PMDRDY*signals being true. Control also proceeds from state MN to state MR4 ifit is a non-burst, memory page hit with a write following a readoperation, as indicated by the MCYC, BURST*, MWR, PGHIT and PRMWR*signals being true. Control proceeds from state MN to state MRC ifanother cycle has been requested and it is a non-burst, non-page hit, asindicated by the MCYC, BURST* and PHGHIT* signals being true. Controlproceeds from state MN to state MI if an idle state is needed fornon-burst, non-bus master cycles, non-memory cycles either because thememory performance monitor has indicated that the next cycle probablywill not be a page hit and therefore it is appropriate to proceed toprecharge the DRAMs to see if time can be saved in the case of a pagemiss, or if a hold request has been acknowledged by the processor 152.These conditions are indicated by the MCYC*, the BURST* signal, and theBMPROG* signals being true and either the HHLDA or MPM₋₋ HIT*. In allother cases control remains at state MN, which is thus effectively asecond idle state, only in this case the MRAS* signal is held low.Particularly, if the memory performance monitor indicated that the nextstate is likely to be a page hit, control remains at state MN, awaitingthe hit and a transfer to states MC1 or MR4.

As noted, the MEM state machine needs to know whether a burst isoccurring. This information is provided by the BURST state machine shownin FIG. 38. This state machine beings operation at state A upon resetand proceeds to state B if a cache burst operation is occurring and anew cycle is being developed, based on the NEWCYC and CBURST signals.The NEWCYC signal is the AND of the MACK and MCYC signals, while theCBURST signal indicates a processor 152 write-back or L2 cache 154 sendallocate line fill. In all other cases control remains at state A.Control proceeds from state B to state C when the MBRDY signal isreceived to indicate that 128 bits of data had been transferred to thememory system. Otherwise control remains at state B. Control proceedsfrom state C back to state A on the next occurrence of the MBRDY signaland otherwise remains at state C. Thus, once the beginning of a newburst cycle is indicated, control proceeds from state B to state C asdata is transferred. The BURST signal is thus asserted during states Band C, while the BURST2 signal is asserted in state C and the EBURSTsignal is provided in state B.

The CHGTO or precharge timeout signal is provided by a simple counter.The counter is loaded with a default value of 4 whenever the MRAS signalis being asserted. Assertion of this signal is described below. On eachrising edge of the PCLK signal when the MRAS signal is deasserted, thatis the MRAS* signal is high, the counter counts down. When the counterreaches a value of 1, this is an indication that the precharge time hasbeen completed for 60 ns DRAMs. When the counter counts down to 0, thisis an indication that the precharge time has completed to 80 ns DRAMs.Therefore the CHGTO signal is provided after three counts for 60 nsDRAMs and four counts for 80 ns DRAMs from the beginning of the MRAS*signal deassertion.

The PGHIT or page hit signal is provided by a page hit detection block.This is relatively conventional block in that it contains a latch forprocessor address <27 . . 12> bits, with the latch operating on theprocessor memory address latch enable signal, which is active whenaddresses are being provided to the memories. The outputs of the latchare provided to one input of a comparator, while the active or currentprocessor address <27 . . 12> bits are provided to the second input ofthe comparator. If they are equal, this is considered to be a page hitoperation, so that a new row address need not be provided.

Certain miscellaneous logic is also contained in the CPUMEM module 430.The first function is the generation of the MRDY signal (FIG. 38A). TheMRDY signal is provided as the output of a D-type flip-flop 740 clockedby the PCLK signal. The D input to the flip-flop 740 receives the outputsignal of a multiplexor 742, with the select input receiving the MWRsignal. The zero input of the multiplexor 742 is connected to the outputof an AND gate 744 which receives the EBURST* signal and a signalindicating that the next state of the MEM state machine is state MC2.The output of the AND gate 744 is also provided to the D input of aD-type flip-flop 746, which is clocked by the PCLK signal and whoseoutput is the LMRDY or EMACK signal. The one input of the multiplexor742 receives a signal indicating that the next state is MC3. This samesignal is provided to the D input of a D-type flip-flop 748, which isclocked by the PCLK signal and whose output is the MBRDY signal.

The MACK or memory acknowledge signal is developed as shown in FIG. 38B.A signal indicating that the next state is state MN and the EBURST*signal are provided to the input of an AND gate 750. The output of theAND gate 750 is one input to an OR gate 752 whose other input is asignal indicating that the next state is state MI. The output of the ORgate is the NMACK signal is provided to the D input of a flip-flop 754.The flip-flop 754 is clocked by the PCLK signal and has the MACK signalas its noninverted output.

The MRASSC or master RAS set signal from the CPU section is the same asthe signal indicating that the next state is state MR1. The MRASRC ormaster RAS reset from the CPU signal is shown as the output of a threeinput OR gate 756 in FIG. 38C. The next state MC3, MCYC, PGHIT*, andBURST2 signals are provided to an AND gate 758. The BURST*, PGHIT*,MCYC, and next state MN signals were provided to an AND gate 760. Theoutputs of the AND gate 758 and 760 are provided as inputs to the ORgate 756, along with a signal that indicates that the next state of theMEM state machine is state MI. The NMCASC or next cycle master CASsignal from the CPU section is equivalent to a signal indicating thatthe next state is state MC.

The NMALEC or next cycle memory address/enable signal from the CPU isprovided as the output of an OR gate 762 as shown in FIG. 38D. The nextstate MC3 and EBURST* signals required as the inputs to an AND gate 764.The next state MN, EBURST*, and MCYC* signals are provided as the inputto an AND gate 766. The outputs of the AND gate 764 and 766 and a signalindicating that the next state is MI are the inputs to the OR gate 762.

The NMADRINCC or next cycle memory address increment signal from the CPUsection is equivalent to the next state being state MC3 during a BURSToperation. The MALLBESC or memory all byte enables signal from the CPUsection is provided during read cycles or during cache BURST operations.The NMWEC or next cycle memory write enable signal from the CPU sectionis provided as the output of an OR gate 768 as shown in FIG. 38E. Thenext state any MR or any MC signal and the MWR signals are the inputs toan AND gate 770. The next state NM, NXTCYC, PWR, PRMWR, and EBURST*signals are the inputs to an AND gate 772. The next state MN, MWR, andEBURST signals are the inputs to an AND gate 774. The outputs of the ANDgates 770, 772, and 774 are the inputs to the OR gate 768.

The NDMPLE or next cycle memory data to processor data latch enablesignal is equivalent to the next state of the state machine being MC3during read operations. The NDMSELOC or next cycle memory data select aleast significant bit from the processor section signal is provided asthe output of an OR gate 776 as shown in FIG. 38F. The next state MC3and next state MN signals are provided as inputs to an OR gate 778,whose output is provided to an AND gate 780, whose other input receivesthe EBURST signal. The next state MC1 and next state MC2 signals are theinputs to an OR gate 782 whose output is an input to an AND gate 784,whose other input receives the BURST2 signal. The outputs of the ANDgates 780 and 784 are the inputs to the OR gate 776.

The NMDOEC or next cycle memory data output enable signal from the CPUsection signal is provided by the output of a NOR gate 786, as shown inFIG. 38G. The next state any MR or any MC state signal and theindication that the machine is in state MC3 are provided as the inputsto an OR gate 788. The output the OR gate 788 is provided as an input toan AND gate 790, whose other input receives the MWR* signal. The nextstate NM, EBURST, and MWR* signals are the inputs to an AND gate 792.The next state MN, NXTCYC, PWR*, and EBURST* signals are the inputs toan AND gate 794. The outputs of the AND gates 790, 792, and 794 and theRDHLD signal are inputs to the NOR gate 786. The RDHLD signal isprovided as the noninverted output of D-type flip-flop 796 which isclocked by the PCLK signal. The state MC3, PRMWR* and EBURST* signalsare provided to an AND gate 798, whose output is connected to the Dinput of the flip-flop 796.

The IMALE signal is a one PCLK delayed version of the MACK signal. Thesignal is used to latch the PWR signal to develop the MWR signal, tolatch the PDC signal to develop the MDC signal and to latch the SPD60 ormemory speed 60 signal to develop the LSPD60 signal.

The PMALE or processor memory address latch signal used with the pagehit detector is developed when the HHLDA signal is low, the MACK signalis true, the MCYC signal is true. The NXTCYC signal is produced as theoutput of an OR gate 800 as shown in FIG. 38H. The SPADS and PMIOsignals are provided as the inputs to an AND gate 802, whose output isone input to the OR gate 800, with the MCYC signal being the other inputto the OR gate 800.

There are also additional types of bus masters which could operate inthe computer system C. One of these is an ISA or Industry StandardArchitecture bus master which would be located in the EISA slots 108. Asthese bus masters can address main memory, it is appropriate to have acontrol module, ISABM 432, and a state machine for use with theiroperation. The state machine is the ISA state machine of FIG. 39. Thestate machine starts at state II upon reset. This is the ISA idle state.When an ISA command is not active, control remains at state II. When anISA command is active, with the definition of the signal defined below,control proceeds from state II to state IR2. Control proceeds from stateIR2 to state IR3 on the next PCLK signal clock edge and then from stateIR3 to state IR4. If a snoop write-back is occurring, that is thewrite-back cache in the processor 152 has determined that a read hit hasoccurred and a write-back is appropriate, control remains at state IR4until this operation is completed. When the snoop write-back iscompleted, control proceeds from state IR4 to state IC1. Then, onsuccessive positive edges of the PCLK signal, control proceeds fromstate IC1 to state IC2 to state IC3 and then to state IL. Controlremains in state IL as long as the ISACMD signal is activated. Controlproceeds from state IL to state II when the ISA command has beencompleted.

The development of the ISACMD signal is shown in FIG. 40. The MRDC andMWTC signals are provided as two inputs to an OR gate 600. The output ofthe OR gate is one input to a four input AND gate 602. The other inputsto the AND gate 602 are the HHLDA, REFRESH* and EMSTR16 signals. Thusthe output of the AND gate 602 is active when an ISA bus master hascontrol of the EISA bus E and is actively running a cycle. The output ofthe AND gate 602 is the XISACMD signal and is provided to the D input ofa D-type flip-flop 604. The inverted clock input of the flip-flop 604receives the HCLK or host bus clock signal, which is developed bydividing the PCLK signal by two for a 66 MHz PCLK signal. The output ofthe AND gate 602 is also provided to D input of a D-type flip-flop 606,whose positive clock input receives the HCLK signal from the host bus H.The noninverted outputs of the flip-flops 604 and 606 are the zero andone inputs, respectively, of a 2:1 multiplexor 608. The select input tothe multiplexor 608 is provided by the PHASE signal, which indicatesthat the PCLK signal is in phase with the HCLK signal, that is, the tworising edges are aligned. The PHASE signal is the noninverted output ofa D-type flip-flop which is clocked by the PCLK signal and receives theHCLK* signal at its D input. The output of the multiplexor 608 isprovided to the D input of a D-type flip-flop 610 whose clocking inputreceives the PCLK signal. The noninverted output of the flip-flop 610 isthe ISACMD signal.

As with memory state machine, the ISA state machine also provides thesignals for controlling the address/control buffer chips 168 and thedata buffer 158. The MRASSI or master RAS set ISA signal is active whenthe ISA state machine is in state II, the ISACMD signal is true and thecycle is being addressed to the main memory. The MRASRI or master RASreset by ISA signal is asserted in state IL when the ISACMD signal isnot asserted. The NMCASI or next cycle master CAS by ISA signal isasserted when any of the next states is state IC1, IC2 and IC3, when theoperation is to main memory and the operation is not to write protectedmemory or is a read operation. The NMWEI or next state MWE from the ISAportion signal is asserted when the next state will be IR4 or one of theIC1, IC2 or IC3 states, an ISA write operation is being requested and itis not to write protected space. The MALLBESI or memory all byte enablesby ISA signal is asserted during read cycles when the ISACMD signal isasserted. The NDMHLEI or next memory data to host data latch enablesignal from the ISA system signal is provided during writes in state IR2or during reads in state IC2. The DHOEI or host data output enable byISA signal is provided during ISA read operations when the ISACMD signalis asserted. The NMDOEI signal is negated when for read operations thenext state will be the IR4, IC1, IC2 or IC3 states, the current state isstate IC3, or the RDHLD signal is asserted.

The ISABM logic 432 is also responsible for providing the CHRDY signal.The logic for this is shown in FIG. 41. The SNPWB signal is provided tothe D input of a D-type flip-flop 620. The PCLK signal clocks theflip-flop 620. The synchronous clear input receives the output of a twoinput OR gate 622 which receives as its inputs signals indicating thatthe ISA state machine is in state II or state IC3. The noninvertedoutput of the flip-flop 620 is provided as one input to a three inputAND gate 624, whose other two inputs receive the indication that thestate machine is in state IC2 and that a read operation is occurring.The output of the AND gate 624 is provided to the load input to a 7 to 0down counter 626. The PCLK signal clocks the counter 626. The output ofthe counter 626 is a zero signal, which is referred to as the CHRDYTO orCHRDY time out signal. This signal is provided as one input to a twoinput AND gate 628, with the other input receiving an indication thatthe state machine is in state IL. The XISACMD signal and the HCW* or notwrite-back signal are provided as the inputs to a two input AND gate630. The XISACMD, signal and an indication of state II are provided asthe inputs to a two input AND gate 632. A two input AND gate 634receives the output of an OR gate 636 and the output of a NAND gate 638.The inputs to the NAND gate 638 are the ISARD signal, indicating a readoperation by an ISA bus master, and the output of the flip-flop 620. Theinputs to the OR gate 636 are indications that the ISA state machine isin state IC1, IC2 or IC3. The outputs of the AND gates 628, 630, 632 and634 are provided as the inputs to an OR gate 640, whose output is theCHRDY signal.

Just as an ISA bus master has a separate control module 432, an EISABMmodule 434 is provided and it includes a state machine referred to asthe EISA state machine which is shown in FIG. 42. The state machinebegins operation after reset at state EI or EISA idle. It remains inthis state until the EISA START or ESTRT signal is received. The ESTRTsignal is true when the START, SBCLK, HMIO, REFRESH*, EMSTR16* and HHLDAsignals are true and the PCLK signal is in phase with the HCLK signal.At that time it proceeds to state ERS. Control remains at state ERSuntil the PCLK synchronized BCLK or SBCLK signal is not asserted.Control then proceeds to state ERW. Control remains in this state whilethe SBCLK signal is not asserted. Control proceeds from state ERW tostate EW1 in the case of the SBCLK signal being asserted and a writeoperation occurring. Control remains in state EW1 when both the SBCLKsignal and BCLK signals are not asserted, as indicated by the SBCLKHIsignal. When they are both asserted, control proceeds from state EW1 tostate EW2. Control proceeds to state EC1 from EW2 in all cases on thenext PCLK signal cycle. Control proceeds directly from state ERW tostate EC1 when the SBCLK signal is true and a read operation isoccurring.

Control proceeds from state EC1 to state EC2 on the next rising edge ofthe PCLK signal. Control proceeds from state EC2 to state EC3 duringread operations and then to state EST on the next PCLK signal edge.Control proceeds directly from state EC2 to state EST for writeoperations. If a snoop write-back is occurring, control proceeds fromstate EST to state EWB, where it remains during the write-backoperation. When the write-back is completed, control proceeds from stateEWB to state EC1 to reexecute the operation which was interrupted by thewrite-back. If a snoop write-back is not occurring and both the SBCLKand BCLK signals are low, as indicated by the SBCLKLO signal, or if asnoop write-back is not occurring and a write has occurred, controlproceeds from state EST to state EN. In all other cases control remainsat state EST. Control proceeds from state EN to state ENW if thesynchronized EXRDY or SBEXRDY signal is not asserted. The SBEXRDY signalis the EXRDY signal first synchronized to the falling edge of the BCLKsignal and then synchronized to the PCLK signal in phase with the HCLKsignal. Control remains in state ENW when either of the SBCLK and BCLKsignals are asserted and moves to state EN on the SBCLKLO signal.Control proceeds from state EN to state ERW if an EISA burst isoccurring and the operation is ready, as indicated by the SMSBURST andSBEXRDY signals. The SMSBURST signal is the MSBURST signal synchronizedto the PCLK signal when both the BCLK and HCLK signals are low. If aburst is not occurring and the cycle is completed, as indicated by theSBEXRDY signal being high, control proceeds from state EN to state EI toenter idle state.

Just as the ISABM and CPUMEM modules 432 and 430 had to develop thevarious signals for controlling the address/control buffers 166 and thedata buffer 158, so does the EISABM module 434. One signal which isproduced is the HSTRETCH* signal, which is provided to the EISA buscontroller 110 to extend the BCLK signal to provide certain incrementaltiming without having to take a full BCLK wait state. The logic fordeveloping the HSTRETCH* signal is shown in FIG. 43. A D-type flip-flop650 is clocked by the PCLK signal and has its input tied to thenon-inverting output. The state ERW and RESET signals are inputs to atwo input OR gate 652, whose output is provided to the synchronous clearinput of the flip-flop 650. The state EWB indication is provided to thesynchronous set input of the flip-flop 650.

The noninverted output of the flip-flop 650 is also provided to oneinput of a two input AND gate 654, whose second input receives the EC orstate EC1, 2 or 3 indication. The output of the AND gate 654 is providedto the D input of a D-type flip-flop 656. The output of a two input ANDgate 658, which receives the PHASE and PCLK signals, is provided to theclock input of the flip-flop 656. The noninverted output of theflip-flop 656 is provided to one input of a three input OR gate 660,whose other two inputs receive a signal indicating that the next statewill be state EC1, EC2 or EC3 or a signal indicating that the currentstate is state EC1, EC2 or EC3. The output of the OR gate 660 isprovided as one input to a three input AND gate 662. The HWR, andHLOCMEM signals are the remaining inputs to the AND gate 662 to indicatea main memory read. A two input AND gate 664 receives the SNPWB and EI*or EISA state machine not in state EI signals. The output of the ANDgate 664 and the output of the AND gate 662 are the two inputs to an ORgate 666, whose output is provided to the D input of a D-type flip-flop668. The PHASE and PCLK signals are provided to an AND gate 670, whoseoutput clocks the flip-flop 668. The noninverted output of the flip-flop668 and the output of the AND gate 664 are provided as the two inputs toa NOR gate 672, whose output is the HSTRETCH* signal.

The MRASSE or master RAS set by EISA portion signal is provided eitherwhen the next state will be state ERW or the MRASSR signal is beingprovided. The MRASSR signal is the set signal based on the refresh statemachine being in state RB. The refresh state machine will be describedbelow. The MRASRE or master RAS reset by EISA signal is provided wheneither the next state is state EI and the current state is state EN,indicating a transfer to idle, or the MRASSR signal is asserted. TheMRASSR signal indicates that the refresh state machine is in state RCand the SBCLKLO signal is asserted. The NMCASR or next cycle master CASrefresh signal is asserted when the refresh state machine is in statesRA or RB. The NMCASE or next, master CAS by EISA cycle signal isasserted either when the NMCASR signal is asserted or when the nextstate is state EC, a main memory operation occurring and it is eithernot write protected or is a read operation. The NMWEE or next cyclememory write enable by EISA signal is provided when the next state iseither EW2, EC, or EWB, and a not write protected write to main memoryis occurring. The NMALEE signal is provided when the next state is otherthan states EC1, EC2 or EC3. The MALLBESE signal is provided during readoperations when the state is other than EI or ERS or when the refreshstate machine is not in state RI. The NDMHLEE or next cycle MDHD latchenable by EISA signal is provided during state EW1 for reads and stateEC2 for writes. The NDHOEE or next cycle HD output enable by EISA signalis provided during the ERW states which are reads to local memory and isprovided to the input of a latch which has its open or enable period onthe high level of the CMD* signal. The output of the latch is the DHOEEsignal. The NMDOEE or next cycle MD output enable by EISA signal is notasserted when read operations are occurring and the next state is stateERW, EC1, EC2, EC3, or EBW, the state is state EC1, EC2, or EC3 or thestate following an EC1, EC2 or EC3 state, i.e. the first entry intostate EST.

The REFRESH state machine used in the REFSM module 442 is shown in FIG.44. It commences operation after reset in state RI, where it remainsuntil it sees the RSTRT or refresh start signal. The RSTRT signal isprovided when the SBCLK signal, the START signal, the REFRESH signal andthe HHLDA signal are asserted. This is the indication of the beginningof the start portion of a refresh cycle on the EISA bus. When the RSTRTsignal is asserted, control proceeds from state RI to state RW. Controlproceeds to state RA and then to state RB on successive PCLK signalrising edges. Control remains at state RB until the SBCLKHI signal isasserted. At that time it proceeds to state RC and then to state RI.

The MABCON module 436 provides the various MRAS*, MCAS*, MALLBES*, MALE,MWE*, MSELCA*, and MADRINC* signals to the address/control buffers 166.The MRAS or master RAS signal and MRAS* signal are provided at theoutputs of a JK flip-flop which receives at its J input the OR of theMRASSC, MRASSI and MRASSE signals and at its K input receives the OR ofthe MRASRC, MRASRI and MRASRE signals. This flip-flop is clocked by thePCLK signal. The MCAS* signal is provided as the inverted output of aD-type flip-flop clocked by the PCLK signal and receiving at its D inputthe OR of the NMCASC, NMCASI and NMCASE signals. The NMSELCA signal isproduced by the AND of the MRAS signal, the SPD60 signal and a signalindicating bus master cycle is not in progress. This signal is providedto the D input of a flip-flop which is clocked by the output of an ANDgate which receives the MRAS and PCLK signals. The inverted output isthe MSELCA* signal. The MALLBES* signal is provided as the output of amultiplexor which receives as its select input the signal indicatingthat a write-back is not occurring and has as its zero input the outputof a NOR gate which receives the MALLBESE and MALLBESI signals and asits one input the inverse of the MALLBESC signal. The MALE signal isprovided at the output of a D-type flip-flop whose input receives theAND of the NMALEC and NMALEE signals and is clocked by the PCLK signal.The MADRINC* signal is produced at the noninverted output of a D-typeflip-flop which receives the NMADRINCC* signal and is clocked by thePCLK signal. The MWE* signal is produced by the inverted output of aD-type flip-flop which receives at its D input the OR of the NMWEI andNMWEE signals and is clocked by the PCLK signal when no write-back is inprogress.

The MDBCON module 438 provides the various selects, latches and outputenable signals to the data buffer 158 by combining them from variousother sources. The MDSEL<1 . . 0> signals are provided by the output oftwo D-type flip-flops whose inputs receive the NDMSEL<1 . . 0> signalsand are clocked by the PCLK signal. The NDMSEL<1> signal receives asignal indicating that a write-back is not in progress, while theNDMSEL<0> signal receives the output of an AND gate which receives atits inputs the NDMSELOC signal and the BMNOWB* signal. The MDHDLE signalis provided at the non-inverting output of a D-type flip-flop clocked bythe PCLK signal whose D input receives the output of an OR gate whichreceives the NDMHLEI and NDMHLEE signals. The MDPDLE signal is providedat the output of a PCLK signal clocked D-type flip-flop which receivesat its input the NDMPLE signal. The MDOE* signal is provided as theinverted output of a D-type flip-flop which receives at its D input theAND of the NMDOEI and NMDOEE signals and is clocked by an AND gate whoseinputs are the synchronized BMNOWB and PCLK signals. The HDOE* signal isprovided at the output of a NOR gate which receives the DHOEI and DHOEEsignals.

One other function in the MCON module 402 is the SNPCON or snoop controlmodule 440. This module 440 is responsible for providing snoop strobesto be used by the various caches and to provide an indication that awrite-back is occurring. This module 440 utilizes the snoop statemachine shown in FIG. 45. This state machine commences at state SI uponreset. When the PEADS or processor external address signal is notreceived, control remains at state SI. The PEADS signal is provided atthe output of a D-type flip-flop which is clocked by the PCLK signal andreceives at its D input the output of an AND gate which receives theHNCA* signal to indicate a cacheable address and the OR of the SNPSTBIand SNPSNBE signals. The SNPSTBE signal is produced during state ERWwhen the SBCLK signal is asserted. The SNPSTBI signal is asserted whenthe NISACMD signal is true and the ISACMD signal is not true. When it isreceived, control proceeds to state SW1 and then to states SW2 and SW3on succeeding PCLK signal rising edges. If there is no indication that aprocessor cache hit has occurred, control returns to state SI. If a hithas occurred, control proceeds to state SWB1. Control remains at stateSWB1 until the SPADS signal is received. Control then proceeds to stateSWB2. Control remains in this state until the LMRDY signal is received,at which time control proceeds to state SWB3 and from there to state SI.The SNPWB or snoop write-back in progress signal is provided when thestate machine is in states SWB1, SWB2 or SWB3.

Various other miscellaneous signals are produced in the MCON module 402.For example, the BMPROG or bus master in progress signal is produced bythe ORing of the BMPROGI and BMPROGE signals. The BMPROGI signal is theISACMD signal, while the BMPROGE signal is provided when the EISA statemachine is not in state EC or the refresh state machine is not in stateR1. The BMNOWB or bus master in progress and not writing back signal isproduced by the ANDing of the BMPROG bus master in progress and SWBPROG*or synchronized write-back in progress signals. The SWBPROG signal isprovided when the snoop state machine is in state SWB1 or the next stateis state SWB2.

The MDHDSEL<1 . . 0> signals are produced by the HA<3 . . 2> signals,which are produced as follows. The NHALEI and NHALEE signals are ANDedto form the NHALEBM signal. The NHALEE signal is active when the nextstate of the EISA state machine will be state EI, ENW or EN, the stateis EI or ERS or if the state is EC2, a write is occurring and it is nota snoop write-back. The NHALEI signal is the inverse of the ISACMDsignal. The NHALEBM signal in turn is used as the selection input formultiplexors connected to the D inputs of two D-type flip-flops, whichare clocked by the PCLK signal. The one input of the multiplexorsreceives the HA<3> and HA<4> signals from the HA bus as being driven bythe bus master. The zero input of the multiplexors receives thenoninverted output of the flip-flops. The noninverted outputs of theflip-flops is also provided to the inputs of two latches. The OR of theBMLEI and BMLEE signals is provided as the enable input for the latches.The BMLEI signal is asserted when the ISA state machine is in state IR2.The BMLEE signal is asserted when the EISA state machine is in stateERW. The noninverted outputs of the latches are the BM₋₋ HA<3 . . 2>signals, which are also the MDHDSEL<1 . . 0> signals.

The HCON module 404 also includes a number of state machines and modulesto develop its necessary signals. The first of these modules is theHSYNC or host sync module 446. The HSYNC state machine is shown in FIG.46. Control proceeds on reset to state HSI, where control remains whilethe HCYC signal from the FECON module 400 is not received. As soon asthe HCYC signal is received, control proceeds to state HSH, where itremains until the NHSTRT or next cycle host start signal is true or whenphase is not properly asserted, that is, the PCLK and HCLK signals aredo not have their rising edges aligned. If the NHSTRT signal is notasserted and the phase is proper, control proceeds to state HSP. Controlremains in state HSP while the NHSTRT signal is not asserted. If theNHSTRT signal is asserted at the same time as the HCYC signal,indicating another cycle is requested, control returns to state HSH. Ifthe NHSTRT signal is asserted and the HCYC signal is not asserted,control returns to state HSI to idle.

The NHACK or next cycle HACK signal is provided during transfers whenthe next state will be state HSI or when the current state is state HSPand the NHSTRT signal is asserted. The HACK signal is provided at theoutput of a D-type flip-flop clocked by the PCLK signal and receivingthe NHACK signal at its D input. The HREQ or host cycle requested signalis asserted when the next state is the HSH state, during the HSH stateor when the HATO signal is not asserted. The HATO signal indicates thatthe necessary address cycles have been completed.

The next control module utilized in the HCON module 404 is the HOSTmodule 448, which uses the HOST state machine shown in FIG. 47. Thismodule 448 provides the control of signals on the host bus when thememory controller 156 is acting as a bus master. Control commences atstate HTI upon reset and remains in that state while the HREQ signal isnot asserted, indicating that a host bus cycle has not been requested.When the HREQ signal is asserted, control proceeds to state HT1. On thenext rising edge of the PCLK signal, control proceeds to state HT2.Control returns from state HT2 back to state HT1 if the HRDY signal isasserted and the HREQ signal is asserted, indicating that one cycle hascompleted and another is pending. Control returns from state HT2 back tostate HTI when the HRDY signal is asserted indicating that the hostoperation is completed and the HREQ signal is not asserted. Controlproceeds from state HT2 to state HT2I if the HRDY signal is notasserted, the HREQ signal is not asserted and the HNA signal isasserted. This initiates pipelining of the host bus H and is an idlestate. Control proceeds from state HT2 to state HT2P if the HRDY signalis not asserted, the HREQ signal is asserted and the HNA signal isasserted. This is the pipelined and not idle condition. Thus HT2I andHT2P are pipelined states. For all other conditions, control remains atstate HT2.

Control proceeds from state HT2I back to state HT1 when the HRDY signaland the HREQ signal are both present, indicating an exit from pipeliningand a new cycle request. Control Proceeds from state HT2I to state HTIwhen the HRDY signal is received and the HREQ signal is not active.Control returns to state HTI to begin idling. If the HRDY signal is notasserted and the HREQ signal is asserted, control proceeds from stateHT2I to state HT2P. In all other cases control remains at state HT2I.Control proceeds from state HT2P to state HT1P when the HRDY signal isasserted. Otherwise control remains at state H2P. Control returns fromstate HT1P to state HT2P when the HREQ signal is asserted, indicating anew cycle is requested; the HNA signal is asserted, indicating that anew pipelined address can be provided and it is not a special cycle onthe host bus. Control proceeds from state HT1P to state HT2I if the HREQsignal is not asserted and the HNA signal is asserted or if the HREQsignal is asserted, the HNA signal is asserted and its a special cycle.Control proceeds from state HT1P to state HT2 if the HNA signal is notasserted, indicating that pipeline mode is being exited.

A signal referred to as NHADS is asserted when the next state will beHT1 or HT2P and the operation is not to the main memory. The HADS*signal is asserted by the inverted output of a D-type flip-flop whichreceives the NHADS signal at its D input during the PHASE signal and thenoninverted output during the PHASE* signal and which is clocked by thePCLK signal. The NLHADS signal is asserted when the next state is HT1 orHT2P and it is a transfer operation with the main memory. The LHADSsignal is asserted as the noninverted output of a D-type flip-flop whichreceives the NLHADS signal at its D input during the PHASE signal andthe noninverted output during the PHASE* signal and which is clocked bythe PCLK signal.

The HACON or host address control module 452 is used to develop the HBEand HA signals. A state machine referred to as the HADD or host addressstate machine is shown in FIG. 48. The state machine begins operation atstate HAI upon reset. Control proceeds to state HA1 when the HADS signalis asserted and in other cases remains in state HAI. Control remains atstate HA1 while the HATO signal is not asserted, indicating that thefinal host address cycle has not been executed. When the HATO signal isasserted, indicating that the final cycle has been completed, controlproceeds from state HA1 to state HA2. The HATO signal is provided by adown counter which is loaded with a value indicating how many 32 bitcycles must be run to complete the cycle in progress. Control proceedsfrom state HA2 to state HAI when the HADS signal is not asserted andeither the HNA or HRDY signals are asserted. At other times controlremains at state HA2. The NHSTRT signal is asserted when the next stateis HAI or when the present state is HAI. The NHASLE signal or next hostaddress latch enable signal is provided when the next state is HAI andthe NHACK signal is asserted or when the next state is HA2 and the HADSsignal is not asserted. The HALE signal is provided as the noninvertedoutput of a D-type flip-flop clocked by the PCLK signal and receivingthe NHASLE signal at its D input. The HALE signal is used to latch theprocessor cycle definition signals to the host bus and to latch theaddresses in the transceiver latch 160.

The HDCON or host data control module 454 also utilizes a state machine.In this case the state machine is the HDAT or host data state machineshown in FIG. 49. The state machine tracks the host bus cycles withrespect to the data so that the data buffer 158 can be operatedproperly. The state machine begins at state HDI upon reset and remainsin that state while the HADS signal is not asserted. Control proceeds tostate HD1 when then HADS signal is asserted. Control returns from stateHD1 to state HDI when the HDTO signal is asserted, the HRDY signal isasserted and the HADS signal is not asserted, indicating that all of thedata has been transferred and a new cycle is not being requested. TheHDTO signal is similar to the HATO signal, except it counts datatransfers. Different counters are necessary because of pipelining.Control proceeds from state HD1 to state HD2 when the HDTO signal isasserted and the HRDY signal has not been asserted, indicating that thefinal transfer is not completed. Control remains at state HD1 in allother cases. Control proceeds from state HD2 back to state HD1 when theHRDY signal is asserted and the HADS signal is asserted, indicating thatthe previous cycle has completed and a new cycle is being requested.Control proceeds from state HD2 to state HDI when the HRDY signal isasserted and the HADS signal is not asserted, indicating that the cycleis completed and a new cycle is not requested. Control remains at stateHD2 while the HRDY signal is not asserted. The HPROG or host cycle inprogress signal is provided when the next state of the state machinewill be state HD1. The HEND or host cycle ending signal is provided instate HD2.

The HDCON module 454 is also responsible for providing signals to thedata buffer 158 as described. To this end the HDSEL <1 . . 0> signalsare developed. The HDSEL<1> signal is the HHLDA signal. The HDSEL<0>signal is developed by the output of a D-type flip-flop clocked by thePCLK signal when in phase and has a zero value provided to the D inputwhen the host data cycle is in progress, the previous data transfer hascompleted and a cache line fill is in progress or when the host datacycle is in progress, the previous data transfer has counted out, acache line fill is not occurring and the lower 4 byte enables of theprocessor 152 are being utilized. The D-input receives a one value whendata start has been initiated as indicated by the HTDO signal being trueand a host cycle in progress and neither a host cache line fill isoccurring nor the upper four byte enables are to be utilized. Theinverse of the output of the flip-flop is provided to its input when acycle is not occurring and the HRDY signal is asserted. In all othercases the output of the flip-flop remains the same.

The HDOE* signal, which is an AND combination of the HDOEM* signalprovided from the MCON module 402 and a signal referred to as HDOEH*.The HDOEH* signal is provided as the output of a D-type flip-flopclocked by the combination of the PCLK signal and the PHASE signal. If ahost cycle is in progress, the input receives the inverse of the HDWRsignal. If the HEND signal and the HRDY signal are both true, then a oneis provided, so that on the next PCLK edge which is in phase the HDOEH*signal is deasserted. The HDPDLE<1 . . 0> signals are provided at theoutputs of two J-K flip-flops. The flip-flops are clocked by the PCLKsignal. The K inputs to the flip-flops receive the respective outputs ofthe flip-flops. The J inputs to the flip-flops are the ANDed combinationof the PHASE signal, the HERDY signal, a signal referred to as HDLWRand, for bit 1 the DHLO* signal indicating that the higher double wordis being operated on, or for bit 0 the HDLO signal indicating that thelower double word is being operated on. The HDLWR signal is provided atthe output of a D-type flip-flop which is clocked by the PCLK signalANDed with the PHASE signal and the HPROG signal. The input receives theinverse of the HDWR signal.

The HARB or host bus arbitrator 408 utilizes a state machine, the HARBstate machine of FIG. 50. The state machine begins operation upon resetin state A, where it remains until the SHHOLD or synchronized HHOLDsignal is asserted. Control then proceeds to state B. Control remains instate B until the SPHLDA or synchronized PHLDA signal and the FEHLDAsignal are asserted. Control then proceeds to state C. Control proceedsfrom state C to state D when the SHHOLD* signal is asserted. Controlproceeds from state D to state A in all cases. Control proceeds fromstate C to state E when the SWBREQ signal is asserted. In all othercases control remains at state C. Control proceeds from state E to stateF when the SWBREQ* signal is asserted and while the SWBREQ orsynchronized write-back request signal is asserted remains in state E.Control proceeds from state F to state C when the SPHLDA signal and theFEHLDA signals are asserted. Otherwise control remains at state F. TheAHOLD or address hold signal provided to the processor 152 is developedat the output of a D-type flip-flop which receives at its input theoutput of an OR gate which receives the NPAHOLD signal and the CAHOLDsignal. The NPAHOLD signal is asserted when the next state will be stateC, state D, state E or state F. The CAHOLD signal is provided from thecache controller 406 during read allocate cache line fill cycles of theL2 cache 154. The NPHOLD signal is asserted when the next state will bestate B, state C, state D or state F. The PHOLD signal is asserted tothe processor 152 from the output of a D-type flip-flop which receivesat its input the NPHOLD signal and is clocked by the PCLK signal. TheNHHLDA signal is asserted when the next state will be state C, state Eor state F. The HHLDA signal is provided by the output of a D-typeflip-flop which receives at its input the NHHLDA signal and is clockedby the combination of the PCLK and PHASE signals.

FIGS. 52 to 69 are a number of timing diagrams illustrating operation ofnumerous signals according to the memory controller of the presentinvention. In many cases the states of the various state machines areshown to allow coordination with the state machine drawings. FIG. 52illustrates a 64 bit processor to memory page hit write to 60 ns memory.FIG. 53 illustrates a 64 bit processor page hit read from 60 ns memory.FIG. 54 illustrates a 64 bit processor to memory page miss write to 80ns memory. FIG. 55 illustrates a 64 bit processor page miss read from 60ns memory. FIG. 56 illustrates a 64 bit processor page miss read from 80ns memory, for close comparison with FIG. 55. FIG. 57 illustrates a pagehit cache line fill from 60 ns memory. FIG. 58 illustrates a page misswrite-back to 60 ns memory. FIG. 59 illustrates a write to writeprotected memory. FIG. 60 illustrates a 64 bit processor pipelined readfrom the host bus H following a host read, while FIG. 61 illustrates anon-pipelined 64 bit processor write to the host bus H. FIG. 62illustrates concurrent processor write cycles to the memory first and tothe host bus H. FIG. 63 illustrates concurrent processor read cyclesfrom memory. FIG. 64 illustrates concurrent processor write cycles tothe host bus H first and to the memory. FIG. 65 illustrates concurrencyby showing an EISA bus E write, followed by a host bus H write, followedby a memory read and pipelining of the next cycle. FIG. 66 illustratesan ISA bus master write, while FIG. 67 illustrates an ISA bus masterread cycle. FIG. 68 shows an EISA bus master write cycle, while FIG. 69illustrates an EISA bus master read hit to a modified location. Asnoted, these are illustrative samples of timing of the memory controller156 and one skilled in the art could readily develop the remainingpossibilities.

The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape, materials, components, circuit elements, wiring connections andcontacts, as well as in the details of the illustrated circuitry andconstruction and method of operation may be made without departing fromthe spirit of the invention.

We claim:
 1. A memory system for a computer, the computer providingmemory cycles including addresses and control signals, the computerhaving a processor, the memory system comprising:a plurality of dynamicrandom access memory devices, each dynamic random access memory deviceincluding address inputs and control inputs, said control inputsincluding a row address strobe input, a column address strobe input anda write enable input; a memory controller receiving the address andcontrol signals from the computer for producing master row addressstrobe, master column address strobe and master write enable signals anda signal indicating selection of row addresses or column addresses; andmeans receiving the address signals from the computer and said masterrow address strobe, master column address strobe, master write enableand address selection signals and connected to said plurality of dynamicrandom access memory devices for providing address, row address strobe,column address strobe and write enable signals to said dynamic randomaccess memory device inputs, wherein said address, row address strobe,column address strobe and write enable signals are provided to saidplurality of dynamic random access memory devices from a singleintegrated circuit; said means for providing address, row addressstrobe, column address strobe and write enable signals to said dynamicrandom access memory device inputs further including a plurality offlip-flops connected to said dynamic random access memory device inputs,said flip-flops clocked by a common clock signal and receiving saidaddress, row address strobe, column address strobe and write enablesignals, so that said address, row address strobe, column address strobeand write enable signals are provided to said plurality of dynamicrandom access memory devices synchronously by said flip-flops, andwherein said flip-flops are located on said single integrated circuitdevice and provide the final output of said single integrated circuitdevice.
 2. The memory system of claim 1, wherein said means forproviding address, row address strobe, column address strobe and writeenable signals to said dynamic random access memory device inputsincludes a multiplexor, said multiplexor receiving the address signalsprovided by the computer and said address selection signal and providingaddress signals for provision to said dynamic random access memorydevices, so that the computer address signals are provided as row andcolumn addresses to said dynamic random access memory devices,andwherein said multiplexor is located on said single integrated circuitdevice.
 3. The memory system of claim 1, wherein said dynamic randomaccess memory devices are organized in a plurality of banks and thememory system further comprises:means for providing signals indicatingselection of a bank of said dynamic random access memory devices, andwherein said means for providing address, row address strobe, columnaddress strobe and write enable signals to said dynamic random accessmemory devices includes means for combining said bank selection signalsand said master row address selection signal to provide row addressselection signals qualified for each bank of said dynamic random accessmemory devices, and wherein said row address strobe qualifying combiningmeans is located on said single integrated circuit device.
 4. The memorysystem of claim 3, wherein the computer provides signals indicating thedata portion units to be provided,wherein said dynamic random accessmemory devices are organized to provide a plurality of data portions,wherein said means for providing address, row address strobe, columnaddress strobe and write enable signals to said dynamic random accessmemory devices further includes means for combining the data portionunit signals with said master column address signal to provide columnaddress signals qualified for each data portion of said dynamic randomaccess memory devices, and wherein said column address strobe qualifyingcombining means is located on said single integrated circuit device. 5.The memory system of claim 1, wherein the computer provides signalsindicating the data portion units to be provided,wherein said dynamicrandom access memory devices are organized to provide a plurality ofdata portions, wherein said means for providing address, row addressstrobe, column address strobe and write enable signals to said dynamicrandom access memory devices includes means for combining the dataportion unit signals with said master column address signal to providecolumn address signals qualified for each data portion of said dynamicrandom access memory devices, and wherein said column address strobequalifying combining means is located on said single integrated circuitdevice.
 6. The memory system of claim 1, wherein the memory cyclesprovided by the computer include burst cycles,wherein said memorycontroller further produces a signal indicating address increment duringburst cycles, wherein said means for providing address, row addressstrobe, column address strobe and write enable signals to said dynamicrandom access memory devices further includes means for incrementing thevalue of said address signals provided to said dynamic random accessmemory devices when said address increment signal is provided, andwherein said address signal value incrementing means is located on saidsingle integrated circuit device.
 7. A memory system for a computersystem, the computer providing memory cycles including address, controlsignals and signals indicating the data portion units to be provided,wherein the memory cycles provided by the computer include burst cycles,the computer having a processor, the memory system comprising:aplurality of dynamic random access memory devices, each dynamic randomaccess memory device including address inputs and control inputs, saidcontrol inputs including a row address strobe input, a column addressstrobe input and a write enable input, said dynamic random access memorydevices are organized in a plurality of banks and to provide a pluralityof data portions; means for providing signals indicating selection of abank of said dynamic random access memory devices; a memory controllerreceiving the address and control signals from the computer forproducing master row address strobe, master column address strobe andmaster write enable signals, a signal indicating selection of rowaddresses or column addresses and a signal indicating address incrementduring burst cycles; and means receiving the address signals from thecomputer, said master row address strobe, master column address strobe,master write enable, address selection and address increment signalsfrom said memory controller and said bank selection signals andconnected to said plurality of dynamic random access memory devices forproviding address, row address strobe, column address strobe and writeenable signals to said dynamic random access memory device inputs,wherein said address, row address strobe, column address strobe andwrite enable signals are provided to said dynamic random access memorydevices from a final output of a single integrated circuit device,wherein said means for providing address, row address strobe, columnaddress strobe and write enable signals to said dynamic random accessmemory device inputs includes a multiplexor, said multiplexor receivingthe address signals provided by the computer and said address selectionsignal and providing address signals for provision to said dynamicrandom access memory devices, so that the computer address signals areprovided as row and column addresses to said dynamic random accessmemory devices, a plurality of flip-flops having outputs connected tosaid dynamic random access memory device inputs, said flip-flops clockedby a common clock signal and receiving said address, row address strobe,column address strobe and write enable signals, so that said address,row address strobe, column address strobe and write enable signals areprovided synchronously by said flip-flops; means for combining said bankselection signals and said master row address selection signal toprovide row address selection signals qualified for each bank of saiddynamic random access memory devices; means for combining the dataportion unit signals with said master column address signal to providecolumn address signals qualified for each data portion of said dynamicrandom access memory devices; and means for incrementing the value ofsaid address signals provided to said dynamic random access memorydevices when said address increment signal is provided, and wherein saidflip-flops provide said final output of said single integrated circuitdevice.